Proliferative activity in renal cell carcinomas seems to be a significant predictor of prognosis independent of tumor stage and grade; its correlation with tumor type is not as well studied. We performed immunohistochemical analysis with antibodies directed against Ki67 (MIB1), cyclin A, and cyclin E on 44 renal tumors, including 2 metanephric adenomas, 9 oncocytomas, and 33 renal cell carcinomas, including 10 clear cell, 11 papillary, 6 chromophobe, and 6 sarcomatoid tumors. MIB1 and cyclin A stained between 0 and 23% of tumor nuclei. Reactivity for cyclin E was rare. There was a positive correlation between reactivity for MIB1 and for cyclin A (Spearman rank correlation r = .3587). Reactivity for either MIB1 or cyclin A did not correlate with tumor type, stage, or grade, but reactivity for MIB1 was significantly higher in patients older than 60 years than in patients 60 years of age or younger (P = .046). Proliferative activity as defined by either MIB1 or cyclin A is independent of tumor type, grade, or stage. The proliferative activity of benign renal tumors (metanephric adenoma and oncocytoma) was not significantly different than that seen in renal cell carcinoma.