This study was undertaken to elucidate the genetic patterns of the renal cell neoplasms of oncocytosis and to compare them with those found in cases with multiple oncocytomas. Three cases of renal oncocytosis and 6 cases of multiple oncocytomas were analyzed. Fluorescence in situ hybridization analysis was performed with centromeric probes for chromosomes 1, 2, 6, 10, and 17 that are typically lost in chromophobe renal cell carcinoma but not in oncocytoma. Immunohistochemistry for cytokeratin 7, parvalbumin, and S100A1 was performed in all cases. Eleven tumors were present in the 3 kidneys with oncocytosis. One of these was a classic chromophobe renal cell carcinoma. In the other 10, none showed any chromosomal losses, whereas 3 showed gains of all 5 chromosomes and 1 had gains of chromosomes 2 and 10. The chromophobe renal cell carcinoma showed losses of chromosome 1, 6, 10, and 17. Twelve of 14 tumors from the patients with multiple oncocytomas showed no loss of any of the chromosomes 1, 2, 6, 10, or 17 and 2 had loss of chromosome 1. All the tumors from kidneys with renal oncocytosis showed strong parvalbumin immunoreactivity, whereas cytokeratin 7 and S100A1 expression were variable. In summary, the renal cell neoplasms of oncocytosis seem to have distinct morphologic, immunohistochemical, and cytogenetic profiles and likely are a distinct entity, not closely related to oncocytoma or chromophobe renal cell carcinoma.