Malignant histiocytosis, also known as histiocytic medullary reticulosis, is a rare neoplasm of malignant histiocytes. Clinical findings include fever, weight loss, hepatosplenomegaly, lymphadenopathy, anemia, thrombocytopenia, and a rapidly worsening course. Approximately 7 percent of patients have skin manifestations early in the course of their disease. In these patients, diagnosis based on examination of a skin biopsy specimen may be crucial. A case of malignant histiocytosis is presented in which the diagnosis was first suggested by findings on the skin biopsy specimen.