Owing to the unclear and mostly unknown etiology of Langerhans' cell Histiocytosis (LCH) and the unsatisfactory results in treating disseminated LCH a prospective multicentric study DAL-HX 83 was commenced, including 45 different clinics of West-Germany, Austria and Netherlands. From June 1st, 1983 to October 31st, 1986, 97 patients (pts) were involved in this study. 35 pts (9 females, 26 males, medium age 6 2/12 years, age range 0/12-14 2/12 years) suffering from localized disease (28x unifocal bone, 6x isolated skin, 1x isolated lymphnode involvement) were treated by surgery and/or radiation or were just kept in observation. 2 children (1 pt with primary localized bone lesion, 1 child with isolated skin rash) developed a new bone lesion after 1/2 year and 1 1/2 years respectively. 62 pts (33 females, 29 males, medium age 2 years, range 0/12-17 1/2 years) with previously untreated disseminated disease were assigned to 3 different risk groups (A, B and C) and were treated according to a standardized induction and risk adapted maintenance protocol. The whole treatment period was limited to 1 year. 19 pts with multifocal bone involvement (group A, medium age 6 1/2 years) were allocated to regimen A, 30 pts with bone and soft tissue involvement or soft tissue involvement alone (group B, medium age 1 8/12 years) to regimen B and 13 pts with dysfunction of the liver, lungs and/or haematopoietic system (group C, medium age 1 year) to regimen C. So far, 1 pt of group A (19 available pts) developed a new bone lesion after 10 months, another pt a suspicious bone involvement 16 months after diagnosis. A 4 months old girl of group B (27 available pts) died 11 months after diagnosis with progressive organ dysfunction, 2 pts are still alive with recurrent multifocal bone lesions and 1 pt achieved stable 2nd clinical remission after a local relapse (mediastinum). 4 pts of group C (11 available pts) died because of progressive disease between 5 days and 3 years after diagnosis, 3 pts are in partial remission after persistent and recurrent disease episodes. All the others are in clinical remission. The medium observation time of the whole group of pts with disseminated LCH is 1 9/12 years (range 0/12-3 5/12 years). The worst prognostic criteria were found to be the presence of organ dysfunction at diagnosis or its development during the course of disease and the age under two years.(ABSTRACT TRUNCATED AT 400 WORDS)