An international randomized trial in Langerhans cell histiocytosis (LCH) has been initiated by the Histiocyte Society. This report reviews the rationale, design, and progress of LCH-I, which compares etoposide (VP-16) and vinblastine in the treatment of disseminated LCH. Data on the risk of etoposide-associated (therapy-induced) malignancy, in the setting of histiocytosis, are reviewed. The available evidence leads to the recommendation that the study of etoposide in LCH should be continued.