Adrenocortical carcinoma is a rare tumor, and only limited information is available about its natural history and the effects of therapy. We studied 105 patients (75 female and 30 male; mean age, 46 years) with adrenocortical carcinoma who were referred to us between 1963 and 1987. The average duration of symptoms before diagnosis was 8.7 months. At the time of diagnosis, 68 percent of the patients had endocrine symptoms, and 30 percent had distant metastases. Hormonal studies showed that 79 percent of the tumors were functional. Eighty patients underwent surgery, and 59 also received the adrenal cytotoxic agent mitotane. The median disease-free interval after surgery was 12.1 months (range, 1 to 175). Tumor dissemination occurred in 82 percent of the patients, most commonly to the lung, liver, and adjacent organs. The median survival time was 14.5 months (range, less than 1 to 175), and the five-year survival was 22 percent. Age over 40 years and the presence of metastases at the time of diagnosis were the only factors recognized as indicating a poor prognosis. Mitotane controlled hormonal secretion in 75 percent of the patients. Eight mitotane-treated patients had partial tumor regression, but the drug did not have a significant effect on survival. We conclude that adrenocortical carcinoma carries a poor prognosis. Mitotane therapy may offer transient benefits, particularly in controlling endocrine symptoms.