Aminoglutethimide inhibited adrenal steroid synthesis, as assessed by serial estimations of plasma dehydroepiandrosterone sulphate in ten patients with advanced breast cancer. There was a favourable response in seven out of the ten patients, and in four there was an objective remission using UICC criteria. Favourable responses were more common in patients who had shown similar responses to previous endrocrine therapy, especially a response to tamoxifen. Side effects were mild: two patients exhibited a drug sensitivity reaction, but neither required cessation of therapy. Adrenal suppression by aminoglutethimide is effective and well tolerated, and is therefore preferable to surgical adrenalectomy in the treatment of advanced breast cancer.