Chemotherapy for children with aggressive fibromatosis and Langerhans' cell histiocytosis. Two disorders involving histologically benign proliferations of fibrous tissue or of histiocytes occur preferentially in children and often require combined management by an orthopedic surgeon and a pediatric oncologist. Treatment of young people with aggressive fibromatosis usually begins with wide local excision of the lesion. However, some tumors cannot be completely removed either because of their location or because of the risk of subsequent serious dysfunction. Not infrequently, local recurrence supervenes despite previous wide local excision, and sometimes multiple tumors are present. In these situations a trial of multiple-agent chemotherapy incorporating vincristine, actinomycin D, and cyclophosphamide may be indicated in an attempt to control the disease. Radiation therapy may also be useful, but the relatively high dose (5000 cGy or more) needed in a growing child is at times a less attractive alternative. Biopsy of a lytic bone lesion in young patients with Langerhans' cell histiocytosis, formerly known as histiocytosis X, is also indicated for initial diagnosis. Biopsy and curettage are usually curative in the patient with an isolated lesion. Patients with multiple simultaneous or recurrent lesions need chemotherapy if dysfunction of the liver, spleen, or lungs is present. Drug therapy may also be beneficial for children with systemic symptoms. This article outlines suggestions for chemotherapeutic treatment in both diseases. 