A randomized trial was performed to determine relative efficacy and toxicity of two first-line combination chemotherapy regimens in women with metastatic breast cancer: CFP (cyclophosphamide, 5-fluorouracil, prednisone) and CMFP (cyclophosphamide, 5-fluorouracil, methotrexate, prednisone). Both regimens have reported efficacy in this setting but differ in dosages and scheduling of the agents they have in common. Three hundred thirty-six women with no prior chemotherapy for metastatic disease were eligible and evaluable, and 309 had either measurable or evaluable disease and were assessable for objective response. Responses were seen in 65 of 153 (42%) on CFP and 83 of 156 (53%) on CMFP (two-sided P = 0.06). Median durations of response were 7.1 months for CFP and 8.5 months for CMFP (log-rank, two-sided P = 0.67). Considering all 336 patients, the median times to disease progression were 4.7 months for CFP and 6.2 months for CMFP (log-rank P = 0.31) and median survivals were 15.2 and 14.9 months, respectively (log-rank P = 0.88). Covariate analysis did not alter these findings. Median leukocyte nadirs were 1800 for CFP and 1500 for CMFP, with 22% and 21%, respectively, having nadirs less than 1000/microliters. Emesis was more frequent on CFP (49%) than on CMFP (26%) but was severe in only 7% and 5%, respectively. It is concluded that despite a higher response rate on CMFP and some differences in toxicities including a higher reported incidence of emesis on CFP, there was no substantial difference in efficacy or tolerability between the two regimens.