Tamoxifen citrate (Nolvadex [Great Britain]; no comparable US product) is a recently developed antiestrogen with significant activity against metastatic breast cancer in postmenopausal women. We investigated its usefulness in breast cancer patients after conventional endocrine therapy and combination chemotherapy had failed. Of the 50 evaluable patients, four (8%) achieved a complete remission, 14 (28%) achieved a partial remission, and ten showed a less than partial response or stabilization of their disease. Median duration of response was eight months, and the survival of responders was significantly prolonged compared to that of the nonresponders. Side effects of the treatment were mild. These results demonstrate that tamoxifen offers the best choice of therapy for patients with metastatic breast cancer after conventional endocrine therapy and combination chemotherapy have failed.