Report on a case of histiocytosis X of the lungs and kidneys. The pulmonary changes occured mainly during the fibrotic phase of the illness. In the kidneys, the process had a distinct granulomatous character with direct transition into a scarring final stage. Unusual was the considerable vascularisation of the pulmonary foci. Because of the large amount of plasma cells, particularly in the renal cortex, an allergic-hyperergic pathogenesis of the illness had to be discussed.