In this phase II trial we have evaluated the activity and toxicity of a combination regimen containing mitoxantrone, L-leucovorin, and fluorouracil in patients with advanced breast cancer pretreated with anthracyclines. Forty-six patients were included into the study; they received a total of 227 cycles of chemotherapy. Median age was 63 years (range 34-78), median performance status was 80 (range 60-100). Visceral metastases were present in 37 patients, 6 patients had bone involvement only, while 3 patients had soft tissue/lymph node disease. Median number of previous chemotherapy regimens for advanced disease was 2 (range 1-3). Ten patients had anthracycline primary resistance (progressive disease during treatment). Twenty-three patients received mitoxantrone 12 mg/sqm day 1; fluorouracil 370 mg/sqm and L-folinic acid 100 mg/sqm days 1-3 administered every three weeks. Another group of 23 patients were treated with the same regimen using a prolonged 5FU/L-FA schedule (5 days). Two complete responses and 6 partial responses were recorded with the 3-day schedule; 7 partial responses in the 5-day schedule (overall response rate 32.6%, 95% C.I. 19-46%). Two partial responses were observed in patients with anthracycline primary resistance. Median response duration was 9 months (range 3-16). Hematologic toxicity was mild: grade 3-4 leukopenia was recorded in 5 patients, grade 3-4 thrombocytopenia in 3 patients. Grade III-IV stomatitis and diarrhea was recorded in 4 and 5 patients respectively (all receiving the 5-day 5-FU/L-FA schedule). Cardiac toxicity was observed in two cases. This regimen proved active in advanced breast cancer following anthracycline-containing chemotherapy, and the 3-day schedule could be offered to such patients with acceptable toxicity.