Summary: Sixty-two patients with histiocytosis X were followed for an average of 5 years. The patients were classified into three groups: general visceral types (14 cases), multiple eosinophilic granulomas (nine cases), and solitary eosinophilic granulomas (39 cases). One hundred bony lesions were noted in 60 of the 62 patients. The bone lesions showed progressive improvement in single and multiple eosinophilic granulomas independent of treatment type. After biopsy, patients received no treatment unless there was a dangerous extension into the soft tissues because of its site, i.e., in the skull. In the general visceral types, chemotherapy was effective in visceral sites and in extensions of the tumor outside the bone but did not alter the natural history of the bony lesion.