INTRODUCTION: This phase II trial was conducted to assess the efficacy and safety of 10-Ethyl-10-Deaza-Aminopterin (10-EDAM), a folate antagonist, in metastatic breast cancer patients who had received no more than one prior chemotherapy regimen. METHODS: Fifty-five patients were treated on an initial weekly dose 80 mg/m(2) of 10-EDAM. Patients who had received a prior chemotherapy regimen in the adjuvant setting (group 1) were considered separately from patients who had received a prior chemotherapy regimen in the metastatic setting (group 2). RESULTS: The response rate for both groups combined was 18%, and median time to progression was 3 months. Median overall survival was 12 months. Treatment was associated with common chemotherapy-related toxicities, such as 25% grade three or four neutropenia and 20% grade three or four stomatitis. CONCLUSION: In patients with metastatic breast cancer who had received one prior chemotherapy regimen, 10-EDAM was well tolerated. In general, while definite antitumor activity was documented, time to progression was brief.