The frequency of Langerhans cell histiocytosis (LCH) and a malignant neoplasm occurring in the same individual appears to be greater than previously recognized. To define the occurrence and the pattern of these events, a Study Group of the Histiocyte Society initiated a registry of patients in whom this association occurred synchronously or asynchronously. Evaluation of 54 patients detected two patterns of associations between LCH and other disorders. First, it is possible that therapy of LCH promotes a secondary malignancy. Second, it is possible that a genetic predisposition, with or without the immunosuppression associated therapy for the malignancy, plays a role in the development and expression of disseminated LCH. Data collected by the LCH-Malignancy Study Group may provide insights into the etiology and pathophysiology of LCH.