Thirty-seven patients with advanced breast cancer were treated with megestrol acetate 160 mg daily. All patients except two had been heavily pre-treated with hormonal therapy; eight patients also received chemotherapy. Complete and partial responses occurred in 25% with a mean duration of 5 months (range 2-24 months). A further 38% of patients had static disease for 2 months or greater. Seven patients had previously received medroxyprogesterone acetate, and responses were seen even in patients who had failed to respond to this therapy. This was thought to be due to the higher levels of progestogenic activity which can be routinely achieved with megestrol acetate. Toxicity was minimal, and we would therefore consider that megestrol acetate should be the progestogen of choice in advanced breast cancer.