From the collection of 2500 cases of renal epithelial tumors in our files, 102 renal oncocytomas were analyzed for size, multifocality and a morphologic spectrum of the growth pattern. The size of the tumors ranged from 1.5 to 13 cm in diameter, with a mean of 6.3 cm. Three cases were multifocal, four cases were combined with another primary renal tumor (1x angiomyolipoma, 1x conventional renal carcinoma, 2x papillary renal cell carcinoma). A central fibrosis or a scar was noted in 13 cases, and there was a gross area of hemorrhage in 11 cases. In 4 cases extensive necroses were recognized. Histologically, an alveolar pattern was noted in 70 cases. A tubular pattern was revealed in 31 cases and an unusual tubopapillar ("glomeruloid") pattern was noted in one case. Foci of atypical nuclei were identified in 58 cases. In 4 oncocytomas broad areas of clearance of the oncocytes were found. Psammoma bodies were recognized in 9 tumors and foci of ossification were present in 4 cases. Intracellular and extracellular hyaline globules were noted in two cases. Renal oncocytoma has a variable morphologic spectrum, and its diagnosis should be based on an analysis of structural and cytologic features. Differential diagnosis of renal oncocytomas with various tumors of the kidney which contain granular cytoplasm is discussed. These tumors with granular cytoplasm include conventional renal cell carcinomas, chromophobe cell carcinomas, and rare examples of papillary renal carcinomas.