DNA ploidy analysis of five renal oncocytomas, six pure oncocytic-granular renal cell carcinomas, 15 pure clear cell renal carcinomas, and two cases of mixed oncocytic-granular and clear cell heterogenous renal cell carcinomas were determined on Feulgen-stained paraffin sections using the (CAS-200 image Analyzer). All five renal oncocytomas were diploid, as were six oncocytic-granular renal cell carcinomas. Three of the 15 clear renal cell carcinomas were aneuploid. In one heterogeneous renal cell carcinoma, the oncocytic-granular foci were diploid and the clear cell focus was aneuploid. The other heterogeneous renal cell carcinoma was uniformly diploid. In 21 renal cell carcinomas, one of 14 stage I tumors was aneuploid, all four stages II and III tumors were diploid, and two of three stage IV cases were aneuploid. All stages I, II, and III patients were free of disease 3 to 48 months after surgery. All three stage IV cases were dead of their disease within 36 months of surgery. We conclude that DNA analysis by image cytometry best identifies the heterogeneity of ploidy patterns in mixed cell type carcinomas, generally correlates with the stage of disease, and may be of value in predicting overall prognosis of renal epithelial neoplasms. DNA analysis by image cytometry, however, did not reveal significant differences between oncocytic granular cell carcinoma and oncocytoma, since both lesions were generally diploid. The distinction between the two tumors is best made on morphologic grounds.