The clinical and endocrine effects of low-dose aminoglutethimide without hydrocortisone in patients with advanced breast cancer were investigated. In a dose escalation study low-dose aminoglutethimide alone (62.5-125 mg twice daily) was as effective as conventional doses with hydrocortisone in lowering serum oestrone and oestradiol concentrations but caused minimum adrenal inhibition, as assessed by serum dehydroepiandrosterone sulphate. 11 of 57 (19%) evaluable patients had tumour regression by objective criteria on this treatment, but the frequency of side-effects was similar to that with conventional doses. Low-dose aminoglutethimide is active in the treatment of breast cancer. It appears to work by inhibition of the aromatase enzyme system in peripheral tissues rather than adrenal suppression.