Twenty-two patients with previously untreated metastatic breast cancer and nineteen patients with refractory metastatic breast cancer were treated with trimetrexate (TMTX). Patients received TMTX 8 mg/m2/day if previously treated or 12 mg/m2/day if previously untreated, both given by intravenous bolus days 1-5, every 21 days. None of the patients previously treated for metastatic disease responded to TMTX. There was one partial responder among the 22 patients with previously untreated metastatic disease. The primary toxicity was hematologic and occurred more frequently in patients with a pleural effusion, low serum protein or albumin, or poor performance status. There were three toxic deaths. The study for previously untreated patients required cyclophosphamide, doxorubicin, and 5-fluorouracil (CAF) after 4 cycles of TMTX. This study design for previously untreated patients allows the Cancer and Leukemia Group B (CALGB) to prospectively evaluate the activity of new agents in "chemotherapy-sensitive" metastatic breast cancer.