In a retrospective study involving 32 haematology/oncology departments in France, 348 cases of Langerhans' cell histiocytosis diagnosed between 1983 and 1993 were collated. The percentage of males was 56.4%. Median age at diagnosis was 30.2 months. The median follow up was 35.5 months. Initially, 108 patients (31%) had isolated unifocal or bifocal bone involvement, 67 (19%) had isolated multifocal bone involvement, 136 (39%) had soft tissue involvement without organ dysfunction, and 37 (11%) had organ dysfunction. Two thirds of the sites of involvement diagnosed throughout the course of the disease were present at diagnosis, while the remaining one third appeared during a relapse. Treatment was tailored to the individual patient and was extremely varied, hampering any comparison of regimens. Vinblastine with or without steroids was the most common regimen when systemic chemotherapy was used for the first episode (246/348). Twenty four of the 216 patients received VP 16 as first line treatment. Two patients with progressive multiorgan relapse, despite the use of several drugs, underwent bone marrow transplantation and are alive and disease free 60 and 22 months later. Altogether 21.9% of patients had sequelae, including diabetes insipidus in 17.5% of cases. The overall survival rate is 91.7% (confidence interval 90.7 to 95%) three years after diagnosis. In the univariate analysis, age less than 1 year, ear, nose, and throat, cutaneous, lymph node, liver, spleen, lung, marrow and intestinal involvement, male sex, progressive episodes, the absence of response, and partial responses, were associated with a poor vital prognosis. In a multivariate analysis of prognostic factors, poor early outcome emerged as the most important parameter, closely linked to other poor outcome features such as young age and organ dysfunction. It identified a small number of patients with a poor initial response to treatment, for whom intensive treatment should be assessed in a phase II trial.