There appears to be a relationship between mitotic activity and malignant behavior in adrenocortical tumors, and carcinomas with a high mitotic index may have a poorer prognosis. This has been investigated further by quantifying and comparing the Ki-67 index using antibody MIB-1 in a series of 14 adrenocortical adenomas and 40 carcinomas. The levels have been correlated with survival and disease-free survival in carcinomas and with evidence of abnormal p53 expression as detected by immunohistochemistry. Nevertheless, many carcinomas have a low level of proliferation that may reflect varying abnormalities within the regulation of both cell division and apoptosis. Expression of bcl-2 protein, an inhibitor of apoptosis has therefore also been examined. The Ki-67 index in carcinomas was significantly higher than in adenomas, but below 4% there was overlap. There was no significant difference in survival between carcinomas with MIB-1 index <3% and those greater, but the lower group had significantly longer disease-free survival (p = 0.02). There was no significant difference between p53 immunopositive and p53 immunonegative carcinomas. No tumor showed immunopositivity for bcl-2 protein. It is concluded that MIB-1 index may contribute additional prognostic information in adrenocortical tumors. Inhibition of apoptosis by bcl-2 does not appear to play a role in tumor growth.