An unusual histiocytic dermatitis associated with a lymphoplasmacytic and histiocytic panniculitis appears to represent a previously undescribed disease process. A 56-year-old woman presented with a 5 year history of a violaceous, maculo-papular rash primarily on her legs. It began approximately 8 months before therapy for thrombocytosis. The rash subsequently progressed to form confluent patches and plaques on her torso and arms. The clinical impression was of disseminated granuloma annulare, but multiple biopsies from different sites did not confirm this. Histologic examination revealed a diffuse proliferation of histiocytic cells in the dermis, without well-formed granulomas or necrobiosis. The cells had large, focally irregular nuclei with prominent nucleoli, and were associated with scattered, often bizarre multinucleate giant cells. Immunohistochemistry demonstrated the histiocytic cells to be positive for S100 and CD68 and negative for CD1a. Perivascular siderophages as well as a lymphoplasmacytic and histiocytic panniculitis accompanied the process. The striking cutaneous changes are of unknown etiology but appear to be previously undescribed.