In a preliminary study we have administered medroxyprogesterone acetate to 20 patients with advanced metastatic prostatic cancer, who no longer responded to previous therapy. The dose used was 1,000 mg intramuscularly three times a week for 5 weeks, followed by 1,000 mg weekly until progression. All patients had undergone orchidectomy or had been treated with estrogens. 15 patients also had received estramustine, and another 3 patients,chemotherapy. A partial response occurred in 4 patients for periods of up to 18 months, and stable disease was seen in 2 patients. 7 more patients reported subjective response lasting less than 2 months. The disease progressed in the remaining 7 patients. Adverse reactions were tolerable. Our results are promising and indicate that medroxyprogesterone in high doses may have an effect in therapy-resistant cases of prostatic carcinoma.