Follow-up examination of a 61-year-old woman who recently had a papillary transitional cell carcinoma removed from her urinary bladder revealed a 5-cm mass in the left adrenal gland. Clinicopathologic studies showed the tumor to be a heretofore undocumented virilizing oncocytic adrenal cortical adenoma. Ultrastructural examination disclosed that the cytoplasm was filled with mitochondria, many of which had tubulovesicular cristae. The smooth endoplasmic reticulum was well developed, as was the Golgi apparatus. Lysosomes of various types, including lipofuscin pigment, were also evident. Immunohistochemical studies revealed variable immunoreactivity for simple epithelial cell cytokeratins and for synaptophysin (weak staining) in a minority of tumor cells. Two additional cases of oncocytic adrenocortical tumors, presumably adenomas, from other institutions are also discussed. The small size and weight of the tumor, the low mitotic rate, and the absence of necrosis and vascular and capsular invasion portend a good prognosis. The patient is currently free of disease 1 year after extirpation of the tumor. Additional cases and longer follow-up times are needed to predict better the behavior of this rare oncocytic adrenal cortical neoplasm.