No effective second-line therapy has been developed for patients with metastatic colorectal cancer progressive after than fluorouracil(FU)-containing chemotherapy. Encouraging results have been reported with weekly 24-hour FU infusion. The aim of this study was to test the activity of weekly FU infusion plus modulation with methotrexate and L-leucovorin in pretreated colorectal cancer patients. Seventeen patients with metastatic colorectal carcinoma were treated with methotrexate (40 mg/m2 on day 1), L-leucovorin (100 mg/m2 as a 4-hour infusion on day 2), and FU (2,600 mg/m2 as a 24-hour infusion on day 2). All patients had measurable disease and a performance status of ECOG O-2. Two of the 17 evaluable patients achieved partial remission (12%; 95% confidence interval 2-24%), 9 had stable disease, and 6 progressive disease. Median time to progression was 4 months (range, 2-8) and median survival was 7 months (range, 3-12+). Toxicity was generally mild or moderate and consisted of mucositis, diarrhea and neutropenia. Pretreated patients with metastatic colorectal, carcinoma benefit only marginally from this treatment schedule.