Biochemical modulation of 5-fluorouracil (5-FU) by folinic acid (FA) increases the response rate in patients with metastatic colorectal cancer compared to 5-FU alone. Phase II trials also demonstrated increased efficacy when interferon was added to 5-FU. In two consecutive trials, 76 patients were treated on days 1-5 with FA 200 mg/m2 plus interferon 5 x 10(6) U/m2 and 5-FU 350 mg/m2 as intravenous bolus injection (n = 33, regimen A) or 5-FU 500 mg/m2 as 2-hour infusion (n = 43, regimen B), repeated every 3 weeks with individual 5-FU dose escalation in steps of 50 (regimen A) or 100 mg/m2 (regimen B). In regimen A 5-FU dose reduction to 300 mg/m2 due to toxicity was necessary in 49% of the patients; in regimen B a 5-FU dose of 600 mg/m2 or above was tolerated by 70% of the patients. Dose-limiting toxicity was severe mucositis and/or diarrhea. Objective responses were observed in 5 of 33 patients (15%) in regimen A (3-28%, 95% confidence interval) and 7 of 41 patients (17%) in regimen B (5-29%, 95% confidence interval). Median time to progression was 4.7 and 4.8 months, and median survival 9.9 and 11.4 months for regimens A and B, respectively. Prolonged 5-FU administration over 2 h allows the administration of a higher 5-FU dose compared to bolus injection with no apparent improvement in antineoplastic efficacy. The addition of interferon to the combination of 5-FU plus FA in this dose and schedule does not seem to improve the response rate but appears to increase treatment toxicity.