Cutaneous histiocytosis may take two principal forms. It is either a benign proliferative process or a relentless, progressive process with a poor prognosis. In histiocytic medullary reticulosis, histiocytes demonstrate nuclear atypia and the outcome is uniformly fatal. Benign cephalic histiocytosis X causes lesions similar to those of histiocytosis X, but Langerhans' cells are absent. In congenital self-healing histiocytosis X, the Letterer-Siwe-like cutaneous infiltrate contains Langerhans' cells, but the lesions heal spontaneously without treatment. The nodular cutaneous lesions of juvenile xanthogranuloma appear in infancy and resolve without treatment; however, the higher percentage (10%) of associated ocular lesions may lead to glaucoma and blindness. In histiocytosis X, the cutaneous lesions show a marked proliferation of Langerhans' cells, with prognosis dependent on the patient's age and the extent of organ dysfunction. Patients who survive the acute form of the disease may develop diabetes insipidus, growth retardation, pulmonary fibrosis, and biliary cirrhosis. A subtle immunologic defect has been identified in patients with histiocytosis X, yet the pathogenesis of the disease is still speculative. Familial disease occurring in early infancy should be differentiated from complete or partial immunodeficiency syndromes. Guidelines for evaluating patients with cutaneous histiocytosis are reviewed.