Figure 1. Typical eruption of erythema nodosum shows erythematous nodules and plaques on the anterior aspects of the legs. This patient had also ulcerative colitis.

Figure 2. Early lesions of erythema nodosum show subcutaneous nodules with a bright red color and raised slightly above the skin. This patient had also Sweet's syndrome.

Figure 3. Close-up view of erythematous nodules of early erythema nodosum. This young woman wan taken contraceptive pills.

Figure 4. A: Nodules may become confluent resulting in erythematous plaques. B: Close-up view of the erythematous plaque on the anterior aspect of the knee. Histopathologic study demonstrated typical erythema nodosum. No underlying disease could be demonstrated in this patient.

Figure 5. Late stage lesions of erythema nodosum show flat plaques with purplish color. This patient had also lung sarcoidosis.

Figure 6. Late stage lesions of erythema nodosum involving the ankle. This patient had also ulcerative colitis.

Figure 7. Late stage lesion of erythema nodosum shows the appearance of a deep bruise ("erythema contusiformis"). This patient had also lung sarcoidosis.

Figure 8. Subacute nodular migratory panniculitis of Vilanova and Piñol. The lesion consisted of a unilateral erythematous plaque that extended peripherally and healed at the center. The progressing border is brightly erythematous, whereas the resolving center has a yellowish hue.

Figure 9. A rare variant of erythema nodosum in children and young adults is characterized by lesions only involving the palms or soles and often the process is unilateral. This boy shows a solitary erythematous nodule on the sole and the histopathologic study demonstrated features of erythema nodosum.

Figure 10. Histopathologic features of an early lesion of erythema nodosum. A: Scanning power shows thickened septa with inflammatory infiltrate. B: Sometimes, in early lesions the inflammatory cell infiltrate may be more apparent in the fat lobules than in the septa, because inflammatory cells extend into the periphery of the fat lobules between individual fat cells in a lace-like fashion, and the process appears as a predominantly lobular panniculitis. C: However, in contrast with authentic lobular panniculitis, necrosis of the adipocytes is not prominent despite the intense neutrophilic infiltrate. D: Higher magnification demonstrates the abundant number of neutrophils. Note the incipient formation of a Miescher's radial granuloma in the upper part of the picture. (A-D, Hematoxylin-eosin stain; original magnifications: A, x20, B, x40, C, x200, D, x400).

Figure 11. Histopathologic features of a fully developed lesion of erythema nodosum. A: Scanning power shows sparse inflammatory infiltrate involving mostly the septa. B: Numerous Miescher's radial granulomas involving the septa. C: Higher magnification shows the characteristic features of Miescher's radial granuloma: Small histiocytes around central clefts of variable shape. D: Still higher magnification demonstrates the palisaded arrangement of the histiocytes around the clefts. (A-D, Hematoxylin-eosin stain; original magnifications: A, x20, B, x40, C, x200, D, x400).

Figure 12. Histopathologic features of a late stage lesion of erythema nodosum. A: Scanning power shows thickened septa. B: Higher magnification shows inflammatory infiltrate involving mostly the septa. C: Still higher magnification demonstrates the abundant number of multinucleated giant cells in the septa. D: Many of the multinucleated giant cells keep in their cytoplasm a stellate central cleft reminiscent of those centers of Miescher's radial granuloma. (A-D, Hematoxylin-eosin stain; original magnifications: A, x20, B, x40, C, x200, D, x400).

Figure 13. Histopathologic features of a late stage lesion of erythema nodosum. This patient had also Crohn's disease A: Scanning power shows thickened septa with inflammatory infiltrate. B: Higher magnification shows thickened septa with inflammatory infiltrate and small size fat lobules. C: Still higher magnification shows that the thickened septa contain abundant blood vessels and inflammatory infiltrate. D: Still higher magnification demonstrates the abundant number of multinucleated giant cells in the septa. E: Still higher magnification shows the multinucleated giant cells surrounding clefts. (A-E, Hematoxylin-eosin stain; original magnifications: A, x20, B, x40, C, x200, D, x400, E, x600).

Figure 14. Histopathologic features of a late stage lesion of erythema nodosum. This patient had also Crohn's disease A: Scanning power shows thickened septa with inflammatory infiltrate. B: Higher magnification shows thickened septa with inflammatory infiltrate that extends to the periphery of the fat lobule. C: Still higher magnification shows the granulomatous nature of the inflammatory infiltrate of the septa. D: Still higher magnification demonstrates the abundant number of multinucleated giant cells in the granulomatous infiltrate of the septa. E: Close-up view of the multinucleated giant cells (A-E, Hematoxylin-eosin stain; original magnifications: A, x20, B, x40, C, x200, D, x400, E, x600).

Figure 15. Histopathologic features of a resolving lesion of erythema nodosum. A: Scanning power shows thickened septa with inflammatory infiltrate at the interface between connective tissue septa and the fat lobules. B: Higher magnification shows thickened septa with inflammatory infiltrate that extends to the periphery of the fat lobule. C: Still higher magnification shows that the inflammatory infiltrate consists of granulation tissue. D: Still higher magnification demonstrates the abundant number of capillary blood vessels and the inflammatory infiltrate at the interface between the septa and the fat lobules. (A-D, Hematoxylin-eosin stain; original magnifications: A, x20, B, x40, C, x200, D, x400).

Figure 16. Erythema induratum of Bazin involving the posterior aspect of the leg of an adult woman with erythrocyanotic circulation and ulcerated lesions.

Figure 17. Histopathologic features of erythema induratum of Bazin. A: Scanning power shows a mostly lobular panniculitis. B: Higher magnification shows thickened septa, but the inflammatory infiltrate involves mostly the fat lobules. C: Still higher magnification shows the inflammatory infiltrate in the fat lobules and the involvement of a blood vessel in the thickened septa. D: Still higher magnification demonstrates that the inflammatory infiltrate involves the vessel wall and obliterates the lumen. (A-D, Hematoxylin-eosin stain; original magnifications: A, x20, B, x40, C, x200, D, x400).

Figure 18. Superficial thromboflebitis. Erythematous nodules in liner arrangement with a cordlike thickening of the subcutis along the involved vein.

Figure 19. Histopathologic features of superficial thrombophlebitis. A: Scanning power shows thrombosis of a large blood vessel within thickened septa. B: Higher magnification shows the inflammatory infiltrate centered in a large blood vessel of the thickened septa. C: Still higher magnification shows that the involved vessel had several muscular fascicles in its wall. D: Still higher magnification demonstrates the inflammatory infiltrate involving the muscular layers of this vein and the obliteration of its lumen. (A-D, Hematoxylin-eosin stain; original magnifications: A, x20, B, x40, C, x200, D, x400).

Figure 20. Cutaneous polyarteritis nodosa presents with tender erythematous nodules on the lower legs that often ulcerate The involved area usually shows livedo reticularis.

Figure 21. Histopathologic features of cutaneous polyarteritis nodosa. A: Scanning power shows sparse inflammatory infiltrate in the subcutis. B: Higher magnification shows the inflammatory infiltrate centered in a blood vessel. C: Still higher magnification shows an eosinophilic ring of fibrinoid necrosis at the tunica intima of the involved blood vessel. D: Still higher magnification of the eosinophilic ring of fibrinoid necrosis giving a targetlike appearance to the vessel. (A-D, Hematoxylin-eosin stain; original magnifications: A, x20, B, x40, C, x200, D, x400).

Figure 22. Subcutaneous sarcoidosis. Erythematous nodule on the forearm. This patient had also lung sarcoidosis.

Figure 23. Histopathologic features of subcutaneous sarcoidosis. A: Low-power view shows the involvement of the fat lobules. Note sparing of the dermis. B: Higher magnification demonstrates the granulomatous nature of the inflammatory infiltrate. C: Still higher magnification shows that each collection of inflammatory cells consisted of small noncaseating granuloma. D: Still higher magnification demonstrates the presence of multinucleated giant cell in the noncaseating granuloma. (A-D, Hematoxylin-eosin stain; original magnifications: A, x40, B, x80, C, x200, D, x400).

Figure 24. Subcutaneous lymphoma mimicking erythema nodosum. Erythematous tender nodules on the lower extremities.

Figure 25. Histopathologic features of subcutaneous "panniculitic" lymphoma. A: Scanning power shows the involvement of both septa and fat lobules. B: Higher magnification shows the infiltrate involving the fat lobules and the thickened septa. C: Still higher magnification demonstrates that the cell infiltrate extends from the septa into the periphery of the fat lobules between individual fat cells in a lace-like fashion. D: Still higher magnification demonstrates atypical lymphocytes with pleomorphic and hyperchromatic nuclei. E: Still higher magnification shows better that these are not inflammatory but neoplastic lymphocytes (A-E, Hematoxylin-eosin stain; original magnifications: A, x20, B, x40, C, x200, D, x300, E, x400).

Figure 26. In elderly patients, especially those with severe venous insufficiency and gravitational edema of the lower extremities, the acute episode of erythema nodosum may be followed by a persistent erythematous swelling of the ankles.