Overall mortality in a series of 42 children with histiocytosis X was 33%. The prognosis was poorer in the younger children and in those whose disease was disseminated from the onset. Direct biopsy of the suspected lesions was the most rewarding diagnostic procedure; bone marrow aspiration supported a correct diagnosis in very few instances. Lahey's scoring system was applied to the series and has been useful for evaluating the patient's status and prognosis. Patients with solitary bone lesions consistently responded satisfactorily to therapy. This was not the case among patients with disseminated disease, in whom the mortality rate was 45%. Therapy with vinblastine sulfate has been quite successful in inducing remissions of the disease's activity, but it remains far from being a definitive and universally effective treatment.