Five renal oncocytomas were studied by light and electron microscopy. Five glutaraldehyde-fixed blocks from each tumor were examined. Scarce short, blunt microvilli, plasmalemmal interdigitations, basement membrane protuberances, and round or oval mitochondrial profiles were found, suggesting that these neoplasms arise from the epithelium of the distal tubule. Morphologic criteria for the diagnosis of renal oncocytoma, including ultrastructural demonstration of cytoplasmic filling by mitochondria, are presented.