A prospective randomized clinical trial was undertaken in 184 patients with metastatic breast carcinoma to compare single drug chemotherapy with L-phenylalanine mustard (L-PAM) and intermittent combination chemotherapy with cyclophosphamide, methotrexate, and 5-fluourouracil (CMF). All patients had not been previously treated with cytotoxic drugs and all had objectively measurable visceral of soft tissue disease. Of the 93 patients who received CMF, 49 (53%) achieved a complete (14 patients) or partial (35 patients) regression of measurable tumor, for a median duration of 25 weeks. Eighteen of the 91 patients (20%) treated with L-PAM responded, for a median duration of 13 weeks. The toxicity was primarily hematologic, and greater in the CMF group, which also received more cycles of therapy because of the higher rate and duration of response. The overall survival of CMF-treated patients was superior to that of the single drug group. The differences were even greater when the patients were subclassified according to the presence of liver involvement or nonambulatory performance status. The superior antitumor effect of CMF over L-PAM suggests that it may be a more effective drug regimen to be used as an adjuvant to primary therapy.