Oncocytomas are usually benign tumors that occur in various organs but particularly in the kidneys. Histologic evaluation of renal oncocytomas shows that they are composed entirely of peculiar epithelial cells with granular eosinophilic cytoplasm. Ultrastructural characterization exhibits densely packed cells with mitochondria, which show morphologic differences from those in normal cells. On the average they are larger than those in renal carcinoma cells and their shape is abnormal (summary by Welter et al., 1989).