The clinicopathological and immunohistochemical features of 91 patients with adrenal cortical carcinoma (ACC) are presented. This series included 52 men and 39 women with a mean age of 45.7 years. According to the extent of cellular pleomorphism, it was possible to define ACC as Grade 1, 2 or 3. There was a correlation between the grading and mitotic activity (P less than 0.05). Median survival of patients with G2 or G3 tumors was only 4.3 months, while that of patients with G1 tumors was 24.3 months (P less than 0.05). The histological grading of ACC was thus shown to be an important prognostic factor. In most cases of ACC, silver impregnation demonstrated zona fasciculata-like structures. Immunohistochemically, vimentin-positive tumor cells could be detected in 41 of 56 cases (73.2%) of ACC, while only 6 of 43 cases (14.0%) of adrenal cortical adenoma showed scattered vimentin-positive cells (p less than 0.01). Considering the fact that the adrenal cortex is of mesodermal origin, this finding may suggest dedifferentiation of the tumor to fetal tissue. Twenty-four of 62 cases of ACC showed cytokeratin-positive cells. Vimentin and silver staining are useful for the diagnostic differentiation of ACC from other carcinomas. In the present study, there was no histologic differentiation between clinically functional and nonfunctional ACC.