A case of urinary malakoplakia involving both ureters and the bladder is presented. Ultrastructural data relating to the ureteric lesion are illustrated. Pale, dark and intermediate cells were involved in the pathological process. Features common and specific for these three types of cell are described. Dark cells are thought to be of a histiocytic nature, while the others are of either histiocyte or urothelial origin. Cytoplasmic inclusions were abundant in all three types in the form of single- or multicentred multilamellar bodies. It was not clear whether these were the product of secretion or phagocytosis. Michaelis-Gutmann bodies, on the other hand, were found to be due to the deposition of needle-like crystals inside pre-existing multicentred multilamellar bodies. Pathogens could not be demonstrated for certain. It is felt, therefore, that the aetiology of malakoplakia must be considered as debatable. Biochemical and immunological research directed to the detection of a possible enzyme deficiency is suggested, along with further ultrastructural investigation of the histogenesis of the pale and intermediate cells.