Histiocytosis X developed in five patients (one woman and four men) when aged between 15 and 44 years. The initial sign in four of them was eosinophilic granuloma of the bone, in one it was pulmonary involvement. In three patients the disease remained confined to bone, while in two it involved the lungs and central nervous system, respectively. Osteolysis regressed spontaneously in one of the men, while in the woman there has been no recurrence 8 years after resection of the focus. In another man an osteolytic focus in a rib was noted after a 9-year recurrence-free interval. The man with pulmonary and bone involvement received chemotherapy with vinblastine and prednisone: dyspnoea and cough disappeared, vital capacity improved and the interstitial lung changes regressed. The osteolytic foci were repeatedly irradiated in the man with bone and CNS involvement. This brought about considerable reduction in pain but no significant radiological changes of the foci. Two courses of chemotherapy were given over 12 years, once with vincristine and prednisone, afterwards with cyclophosphamide. This arrested the progression of the osteolytic foci, but each time they recurred when the drugs were stopped.