A high rate of response to 5-fluorouracil (5FU) and alpha-interferon (alpha IFN) combination therapy has been reported in metastatic colorectal cancer patients. Therefore, designed a trial of high-dose continuous-infusion 5FU, oral leucovorin (LV), and alpha IFN in this group of patients. Because this combination has not previously been tested and severe toxicity has been reported for 5FU and alpha IFN combination therapy, we conducted a phase I trial in which 11 patients presenting with previously untreated metastatic colorectal cancer were treated with escalating doses of alpha IFN together with fixed doses of 5FU and LV. WHO grade III toxicity consisting mainly of oral mucositis was noted in four patients. No grade IV toxicity occurred. Although alpha IFN may enhance the toxicity of 5FU, the toxicity of this regimen remained manageable. Three partial responses were noted.