Seventy-eight advanced breast cancer patients, most of whom had had prior treatment, were treated with the synthetic antiestogen tamoxifen. The overall objective response rate was 27% (21/78). An additional 19% (15/78) showed disease stabilization. Sixty-seven percent (14/21) of the responses were in soft tissue sites, 24% (5/21) on bony sites and one each occurred in liver and nodular lung disease. Forty percent of patients with soft-tissue disease alone responded, while less than 10% of patients with visceral disease showed responses in visceral sites. The response rate was 28% among patients with a known positive estrogen receptor (ER) assay. It was 21% among patients who had previously received cytotoxic drugs. Toxicity was mild and was seen in nausea and vomiting, hot flushes and vaginal bleeding, and occasional myelosuppression. One patient was withdrawn from the study because of a rash. In two patients the disease flared, once with concomitant hypercalcemia. Tamoxifen is a useful agent for advanced breast cancer even in some patients with visceral disease.