Neoplastic erythema nodosum.

We performed immunohistologic studies on a 75-year-old white woman with erythema nodosum (EN) and a systemic lymphoma. A skin biopsy specimen from an EN lesion showed lobular and septal pannicular infiltration by atypical lymphocytes. The cutaneous lymphocytic infiltrate was composed of a monoclonal population of B cells with lambda light chains. Atypical lymphocytes were also seen in the peripheral blood, and flow cytometry showed a predominance of the same phenotype of B cells with lambda light chains. Chemotherapy for systemic B-cell lymphoma resulted in the simultaneous resolution of EN and the lymphoma. This is the first documentation of EN representing direct cutaneous invasion by a B-cell lymphoma.