The antitumor effect and side effects of oral high-dose Medroxyprogesterone acetate (MPA) therapy were studied in 110 patients with advanced or recurrent breast cancer. MPA in 200 mg tablets was consecutively administered at a daily dose of 600, 800, 1,000, 1,200, 1,600, 1,800 or 2,400 mg. The overall response rate was 32/110 (29.1%), and the highest response rate was obtained in the 1,200 mg group (38.2%). The response rate by the site of lesion was higher in the soft tissues and also in bone metastases. Side effects such as moon face, vaginal spotting and abnormal glucose tolerance was mild and tolerable. Accordingly, from the antitumor effect and side effects, the optimal dose was considered to be 1,200 mg a day. Furthermore, as MPA therapy increases appetite and body weight of patients without causing myelopoietic depression, this drug could be used successfully in combination with other chemotherapeutic agents.