Renal oncocytoma.

The author reports on a case of renal oncocytoma, showing up as a side finding during autopsy. In the light microscopic picture of the tumour typical tubular alveolar, in some places solid arrangement was observed. The cytoplasm of the tumour cells could be characterized by even, subtle granulation. Malignant transformation was detected. It has been proved by electron microscopic examination that the granular appearance of the cytoplasm originated from mitochondria. In the light of the literary data the author underlines the rarity and the diagnostical importance of the alteration. Oncocytoma should be distinguished from granular, cellular cancer of the kidney and from malignant oncocytoma.