A 26-year-old man with a history of Langerhans'-cell histiocytosis (LCH) of the bone presented with nephrotic-range proteinuria. Renal biopsy results showed changes characteristic of membranous nephropathy. During the current hospitalization, the patient had 2 episodes of pulmonary embolism. LCH at this time was documented in the lymph nodes. The patient was treated with repeated courses of vinblastine and high doses of corticosteroids to achieve remission of the basic disease and the renal involvement. After 2 years, complete remission of both the lymphadenopathy and the nephrotic syndrome was achieved. The association of membranous nephropathy with LCH might be attributable to an underlying abnormality in the immune system, a paraneoplastic manifestation, or both. This is the first report of LCH associated with severe nephrotic syndrome caused by membranous nephropathy.