A group of 122 postmenopausal patients with histologically proven node-positive primary breast cancer have been randomized to receive aminoglutethimide-hydrocortisone or placebo aminoglutethimide-placebo hydrocortisone for 2 years. Median follow-up is 17 months. In general, treatment was well tolerated, but 15 patients required a reduction in the dose of aminoglutethimide, and of these four patients were unable to continue therapy due to side effects. Primary staging, incidence of extensive node involvement, and estrogen receptor were similar in the treatment and control arms. Dehydroepiandrosterone sulfate (DHA-S) and estrone were measured in a subgroup of patients, and significant suppression of DHA-S levels throughout the duration of the treatment period as seen in patients receiving the active drug. No significant suppression of either DHA-S or estrone levels was seen in the controls. Patients were monitored for metastases by serial liver function tests, carcinoembryonic antigen, and chest X-rays, and of 26 relapsing patients only three patients were not detected by this screen. We conclude that adjuvant aminoglutethimide is moderately well tolerated. It is capable of suppressing DHA-S throughout 2 years of treatment. A further 280 patients will be entered into the study to assess the survival benefit for those taking aminoglutethimide-hydrocortisone.