The pathologic materials and clinical courses of 36 children aged 1 month-22 years, with histiocytosis × (H-X) seen at the Philadelphia Children's Cancer Research Center from 1970 to 1979 were reviewed. The pathologic subtype of H-X was favorable (type II) in 31 patients, unfavorable (type I) in one patient, and unclassified in four patients whose specimens were limited to a skin biopsy. Sixteen patients had localized H-X involving bone (14 patients), soft tissue (1 patient), or skin only (1 patient); all are alive and well after treatment with surgery alone (12 patients), radiation therapy (RT) (3 patients), or observation (1 patient); only 1 of the 16 developed recurrent H-X. The other 20 patients presented with multifocal H-X involving the skeleton alone (3 patients); the skeleton and soft tissues other than liver (7 patients); soft tissue exclusive of the liver (3 patients); soft tissue including the liver (4 patients); or soft tissues, skeleton, and liver or multiple drugs ± RT (15 patients). Seven of the 20 patients are alive and well without recurrence at a median of 4 years after diagnosis. Nine of the 20 patients, including 3 with liver dysfunction, responded completely to initial therapy but developed recurrence; each was retreated with drugs and is alive and well at a median of 4 years. The remaining 4 patients had widespread disease with dysfunction of the liver and/or hematopoietic system at diagnosis, failed to respond, and died. We conclude that (1) patients with multiple bony lesions with or without associated soft tissue disease or skin involvement have a favorable outlook and do not require systemic chemotherapy; (2) systemic treatment also is unnecessary for patients with localized H-X since recurrence is rare; (3) drugs can benefit patients with multifocal H-X, although the optimal duration of therapy is unclear; and (4) favorable response to treatment indicates high probability of disease-free survival. However, organ dysfunction at diagnosis is ominous: four of seven patients with liver dysfunction are dead, as are all three patients who prsented with peripheral blood count depression.