We present the clinical, histologic, immunohistochemical, and ultrastructural findings of four cases of non-functioning oncocytic adrenocortical carcinomas. The patients' ages ranged from 39 to 71 years. There was no sex predilection. Large yellow-tan tumors (8.5 to 17.0 cm), well demarcated from the adjacent kidney, were seen with a thin rim of normal adrenal gland along one edge. One tumor invaded the inferior vena cava and extended up to the level of the right atrium, and another metastasized to bone. The other two tumors had similar morphologic features and therefore were considered carcinomas. Histologic sections of all four cases showed a diffuse proliferation of polygonal neoplastic cells with large nuclei containing prominent nucleoli and abundant granular and eosinophilic cytoplasm. Occasional mononuclear and binucleated giant cells were noted in one case. There were rare mitotic figures (less than one per 10 high power fields). All tumors were immunoreactive for cytokeratins (AE1/AE3 and CAM5.2). Inhibin was focally expressed by one tumor and its bone metastasis. Ultrastructurally, the cytoplasm of the neoplastic cells was packed with innumerable mitochondria. Cytologic atypia or mitotic rate cannot reliably predict the biologic behavior of oncocytic adrenocortical neoplasms. Large tumor size (4/4), extracapsular extension (3/4), blood vessel invasion (2/4), necrosis (4/4), and metastasis (1/4) are features of malignancy for oncocytic adrenocortical carcinomas. The treatment of these tumors is complete surgical excision.