Eighty-nine postmenopausal women with metastatic breast cancer, in whom estrogen receptors (ER) were positive or unknown, were treated on a controlled trial to determine the effectiveness of tamoxifen and to assess the therapeutic advantage of sequentially adding low-dose cyclophosphamide-methotrexate-5-fluorouracil (CMF) chemotherapy in tamoxifen responders. Patients with known ER negative status were not studied. After the initial 12 week treatment with tamoxifen alone, 59% of ER positive patients achieved complete or partial response as did 35% in whom ER were unknown. Response status further improved in 18% randomized to continue tamoxifen alone vs. 28% in whom CMF was added to tamoxifen. There were no statistically significant differences in time to the development of progressive disease or survival between the ER positive and ER unknown patients or between the tamoxifen and tamoxifen plus CMF groups. We conclude that inability to determine ER status should not prejudice against the use of tamoxifen in postmenopausal patients with advanced breast cancer. As yet, no benefit has been demonstrated from the addition of CMF chemotherapy in tamoxifen responders.