Cyclin D1 plays an important role in cell cycle progression from G1 to S phase. Cyclin D1 overexpression has been identified in many human neoplasms, including a variety of carcinomas. A systematic study of cyclin D1 expression in renal carcinomas and oncocytomas has not been reported. Ninety-six renal epithelial neoplasms, 78 renal carcinomas (45 clear-cell, 18 papillary, and 15 chromophobe), and 18 oncocytomas were analyzed immunohistochemically using routinely fixed tissue sections and a cocktail of two monoclonal anti-cyclin D1 antibodies. One thousand cells were manually counted, and the percentage of cyclin D1 positive cells was calculated. Fluorescence in situ hybridization studies using chromosome 11 centromeric and 11q13 specific probes were performed on a subset of clear-cell carcinomas and oncocytomas. Cyclin D1 immunoreactivity was observed in 23 (51%) of 45 clear-cell, 5 (28%) of 18 papillary, and 2 (13%) of 15 chromophobe carcinomas. Nine (50%) of 18 oncocytomas were positive for cyclin D1. Cyclin D1 expression in clear-cell carcinomas did not correlate with survival. Fluorescence in situ hybridization studies on eight clear-cell carcinomas and seven oncocytomas revealed normal chromosome 11 number and no evidence of amplification of the 11q13 locus. Thus, cyclin D1 can be immunohistochemically demonstrated in approximately one-half of renal oncocytomas and clear-cell carcinomas and is less frequent in papillary and chromophobe carcinomas. The mechanism of cyclin D1 expression is unknown, but it does not seem to be related to extra copies of chromosome 11 or to gene amplification.