Fifty-six cases of adrenal cortical neoplasm with a minimum of 5 years follow-up are presented: 48 carcinomas and 8 adenomas. Adenomas typically had a maximal mitotic rate of fewer than 2 mitotic figures per 10 high-power fields (all cases), a prominent small nest growth pattern (7 cases), predominantly clear or foamy cytoplasm (6 cases), and no tumor necrosis (all cases), whereas carcinomas were characterized by at least 4 mitotic figures (often many more) per 10 high-power fields in the most active area (all cases), lack of a significant small nest growth pattern component (45 cases--solid or trabecular growth most common), at least a considerable proportion of cells with eosinophilic cytoplasm (all cases), and tumor necrosis (45 cases). Carcinomas were almost always larger than adenomas, but two adenomas (5.9 cm and 7 cm) overlapped in size with the four smallest carcinomas (5.5 cm, 6 cm, 7 cm, and 7 cm, respectively). The patients with adenomas were older on the average than those with carcinomas (median 58 years, range 31-71 years versus median 41 years, range 5 months-66 years). Two adenomas and 19 carcinomas were functional. No patient with adenoma had recurrence of tumor after excision, whereas all but nine carcinoma patients died of tumor, after 1 to 183 months. Among carcinoma patients, survival was significantly shorter when distant metastases were manifest at diagnosis (P = .0003). There was a trend toward shorter survival with higher mitotic rates and functional tumors, but neither these nor any other parameter had a statistically significant relationship to survival or tumor behavior when presence/absence of metastases at diagnosis was taken into account.