Forty-three premenopausal women with metastatic breast cancer were treated with tamoxifen (2 X 20 mg daily) as first-line treatment. The patients were not selected on receptor status, which was known in only a few patients. Complete response was achieved in six patients and partial response was achieved in seven patients, for a total response rate of 30%. At the time of disease progression (including nonresponding patients), castration was performed as second-line treatment in 24 patients. Four of eight responders to tamoxifen had a response to castration. Of four patients with stable disease receiving tamoxifen, one had a response, and of 12 patients failing to respond to tamoxifen, one had a complete response and one had a partial response. In conclusion, patients who respond to tamoxifen have a good chance of responding to castration; an initial failure to tamoxifen does not exclude the possibility of response to castration. Therefore, the prognostic value of the tamoxifen response for castration response is of limited value.