Renal cell carcinoma vs. renal oncocytoma. Report of a case with overlap features and review of the literature Although the salient features of renal oncocytomas and renal cell carcinomas have been discussed in the recent literature, renal masses with features of both entities will present diagnostic difficulty, especially when the cells are diffusely eosinophilic on microscopic examination. A case of a firm, tan, rounded mass replacing the lower pole of the kidney is discussed. The final diagnosis of renal cell carcinoma, granular cell type, was made after multiple sections of the tumor were examined, and after electron microscopy was performed. A thorough search by light microscopy should be made for clear cell foci, necrosis, mitotic activity, and vascular or capsular invasion, features generally accepted as pathognomonic for renal cell carcinoma. Cellular and especially nuclear pleomorphism is typically focal or mild in renal oncocytomas. True oncocytic tumors will be packed with mitochondria on electron microscopy; however, granular renal cell carcinomas will contain mitochondria as well as other cellular organelles, lipid, and glycogen. Electron microscopy should be performed on tumors suspected of being oncocytomas because eosinophilia on hematoxylin and eosin stain, as demonstrated by this case, is not a predictable measure of mitochondria content. Immunoperoxidase staining for vimentin in oncocytomas has recently been shown to be negative, and may offer a method of ruling out oncocytoma in vimentin-positive tumors, pending further studies. 