In a prospective randomized multicenter study in previously untreated postmenopausal patients with advanced breast cancer, the response to treatment with oral medroxyprogesterone acetate (MPA) 300 mg three times daily was compared with tamoxifen (TAM) 20 mg twice daily. Of 61 patients treated with MPA, 27 (44%) had a partial or complete remission, 6 showed no change, and 28 had progressive disease. Of 68 patients treated with TAM, 24 (35%) showed a remission, 15 no change, and 29 progression. The difference in response rate is not significant. However, 11 of 25 patients with osseous metastases as predominant site, responded to MPA and 7 of 31 to TAM (P = 0.05). Moreover, in patients older than age 70 years, 13 of 26 responded to MPA and 6 of 31 to TAM (P less than 0.05). Median duration of remission of all patients in the MPA arm was 17 months and in the TAM arm, 23 months (not significant). Median survival was 20 months for MPA and 26 months for TAM (not significant). After cross-over from TAM to MPA 8 of 31 patients responded and after cross-over from MPA to TAM, no response was seen in 27 patients. These data indicate that the response rate and duration to MPA and TAM are comparable, except in patients with osseous metastases and in patients older than age 70 years. MPA has more side effects, but seems to be more effective after cross-over, and may thus be reserved for second-line treatment.