A pilot study was undertaken to compare mitomycin C plus oral high-dose medroxyprogesterone acetate (MMPA) to cyclophosphamide+doxorubicin+ fluorouracil (CAF). Thirty-four women were randomized at first relapse to receive MMPA or CAF. Patients were balanced with respect to age, performance status, hormone receptor status, prior adjuvant treatment, site of metastases, and number of metastatic sites. On MMPA 9/18 objective responses occurred and on CAF 12/18. Median time to treatment failure was 5.7 months on MMPA and 7.6 months on CAF; median survival on MMPA was 22.5 months and on CAF 16.7 months. Although there were more objective responses on CAF, this was not statistically significantly different, and CAF was associated with significantly more hemopoietic toxicity. It is concluded that mitomycin C should be further studied in front-line regimens for patients with metastatic breast cancer.