Thirty-four patients with metastatic breast cancer (MBC) who had progression of disease after high-dose chemotherapy (HDCT) with peripheral blood progenitor cell support (PBPC) had methotrexate, uracil and tegafur (UFT), and leucovorin (MUL) therapy administered: methotrexate administered intramuscularly in combination with UFT given orally and leucovorin given orally. All patients had received extensive prior chemotherapy including a high-dose regimen with PBPC support. Two complete responses (CR) and 11 partial responses (PR) were observed (objective response rate: 13/34 or 38%, 95% confidence interval 22-56%). Seven additional patients had stable disease (SD), 4 of whom (12% of the total population) of 6 months or longer duration, with the clinical benefit rate (CR + PR + SD of at least 6-month duration) reaching 50%. Median follow-up was 38 months, and the median time to progression and the median overall survival time from the start of MUL were 5.5 and 11 months, respectively. Toxicity was mainly gastrointestinal. Eight patients (24%) had World Health Organization grade II or greater diarrhea and/or enteritis and, consequently, the UFT dose was reduced. Emesis was mild and easily manageable with thiethylperazine given orally. The regimen did not produce significant myelosuppression or alopecia. In conclusion, patients with MBC retain chemosensitivity even when they progress after HDCT/PBPC and can be treated again with chemotherapy. MUL is active and well tolerated in patients with MBC progressing after HDCT. Further studies with this regimen, as salvage chemotherapy or as maintenance chemotherapy after HDCT/PBPC, would appear to be warranted.