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Clinicopathological features of four cases of oncocytic adrenocortical carcinomas were studied. All tumors were large, circumscribed tumors with average size and weight of 11.5 cm and 586 g, respectively. The cut surfaces were yellow or brown and tan with areas of hemorrhage, necrosis, fibrosis, myxoid and cystic change. The tumor cells were exclusively oncocytic with a diffuse or compact and solid arrangement. Nuclear atypia was identified but mitosis was rare. Capsular invasion was identified in all tumors and vascular invasion was identified in one tumor. All tumors were immunoreactive for vimentin and inhibins. Immunoreactivity for pancytokeratin, synaptophysin and S-100 protein was variable and focal. All tumors had low proliferative indices, of less than 1%, and were negative for p53 protein. Ultrastructurally, the cytoplasm of tumor cells showed numerous mitochondria in a compact arrangement. Oncocytic adrenocortical carcinomas showed a similar sex ratio, slightly older mean age, similar left predilection, slightly smaller size and lighter weight compared with the conventional carcinomas. We suggest that most oncocytic adrenocortical carcinomas might be low-grade malignancies with less aggressive histological features compared with conventional carcinomas. However, they should be excised completely because of the likelihood of recurrence and metastasis during the follow-up period.