The clinical data and morphologic findings in 16 cases of adrenocortical carcinoma were compared with those in 11 cases of surgically removed functional adenomas and 12 cases of nonfunctional adenomas found at autopsy. Histopathologic changes of architectural disarray, pleomorphism, increased mitotic activity, vascular invasion, hemorrhage, or necrosis were generally reliable criteria for diagnosis of malignancy. However, weight was the parameter that most consistently correlated with outcome, since all patients with tumors under 50 Gm. survived and all lesions of 95 Gm. or over proved to be malignant.