A phase II trial of the new antifolate edatrexate (10-ethyl-10-deaza-aminopterin) was performed in thirty-eight patients with metastatic breast cancer who had never received chemotherapy. Edatrexate was administered as a weekly intravenous bolus injection at a dose of 80 mg/m2. Sites of metastases included visceral (31%), soft tissue/lymph node/bone (51%), and bone only (18%). Thirty-two patients were evaluable for response; there were 3 complete responses (CR) and 8 partial responses (PR), yielding a response rate (CR plus PR) of 34% (95% confidence limits, 17.9% to 50.9%). Responses were seen in soft tissue metastases, in visceral metastases (liver, lung) and in one patient with bone metastases. Median duration of response was 30 weeks (range 12-66 weeks). Substantial toxicity was observed. The dose-limiting toxicities were mucositis, myelo-suppression and skin toxicity. The general toxicity profile was similar to that usually reported for methotrexate, but mucositis and skin toxicity were more pronounced. Edatrexate appears to be an active drug in the treatment of chemotherapy-native patients with metastatic breast cancer.