Tamoxifen (NSC-180973; ICI-46474) can provide palliation to patients with advanced breast cancer whose tumors contain estrogen-binding proteins (EBP). The drug is most effective in patients with bone metastasis and minimal prior therapy. In the present study, 72 patients with advanced breast cancer were evaluated for their response to oral tamoxifen therapy administered at a dose of 20 mg twice daily. Twenty-eight of 72 patients (38%) demonstrated objective responses to tamoxifen therapy. For patients with a positive EBP and no prior chemotherapy, eight of 11 (74%) responded. No patient possessing a tumor negative for EBP achieved a remission. Patients with tumors that possessed normal arylsulfatase B and glucose-6-phosphate dehydrogenase enzyme activities responded most favorably to tamoxifen therapy. These results demonstrate that tamoxifen is effective in the treatment of patients with advanced breast cancer, and EBP and specific enzymes might be useful in selecting the patients for hormone manipulation.