The endocrine effects of replacement doses of hydrocortisone in postmenopausal women with advanced breast cancer were compared with the same doses of hydrocortisone plus aminoglutethimide. Fifteen patients received aminoglutethimide (AG) 250 mg three times a day plus hydrocortisone (HC) 20 mg twice a day for 2 weeks, then AG was increased to 250 mg four times a day. Another 13 patients received HC alone for 2 weeks, then AG was added. HC alone significantly suppressed oestrone (75% of baseline) and oestradiol (50% of baseline). Addition of AG to these patients produced further oestrone suppression (50% of baseline) significantly greater than HC alone. HC alone suppressed dehydroepiandrosterone sulphate as much as AG + HC. delta 4-androstenedione (delta 4A) and dehydroepiandrosterone (DHA) were suppressed by HC alone. Addition of AG produced a rise of delta 4A to basal levels. These results show that 3-beta-ol de hydrogenase is not induced by AG. AG plus HC together from day 1 produced significantly greater oestrone suppression (50% of baseline) than HC alone. Because high-dose steroids may induce aromatase and replacement doses produced marked peripheral endocrine effects, the use of replacement hydrocortisone should be reassessed in advanced breast cancer.