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Eight conventional and six atypical oncocytomas in a series of 147 renal neoplasms were studied. Histopathologic findings revealed exclusively oncocytic cells, but cellular polymorphism was higher in the atypical tumors. Atypical oncocytomas presented focal necrosis, transcapsular invasion, or both. Electron microscopy showed similar findings in all cases. Immunohistochemistry of atypical oncocytomas had higher expression against proliferating cell nuclear antigen and more discontinuous immunostaining against laminin than typical ones. Flow cytometry revealed one or two aneuploid peaks in five typical and two atypical cases, although the latter had a higher proliferative fraction than typical oncocytomas. Cytogenetics of one typical oncocytoma showed a normal diploid karyotype; one atypical case resulted in a diploid karyotype but with endoreduplications in 13% of metaphases, and a second atypical oncocytoma became hypodiploid without structural anomalies. Based on the present results, the proposed distinction between conventional and atypical oncocytomas seems of limited clinical significance.