In a pilot clinical trial, treatment of patients with advanced colorectal carcinoma with the combination of fluorouracil (5FU) and recombinant interferon alfa-2a (IFN) resulted in objective tumor regression in 62% of patients. To confirm these findings in a multiinstitutional setting, a phase II clinical trial was initiated by the Eastern Cooperative Oncology Group (ECOG) in 1989. The treatment regimen was identical to that used in the earlier study: 5FU 750 mg/m2/d for 5 days as a continuous infusion followed by weekly outpatient bolus therapy and IFN 9MU subcutaneously beginning day 1 and administered three times per week. Doses were modified for gastrointestinal, hematologic, and neurologic toxicity and for fatigue, similarly to those used in the previous pilot trial. Thirty-eight patients were registered; 36 are evaluable for response (one lost to follow-up and one with nonmeasurable disease). All patients had metastatic or locally recurrent disease beyond the scope of resection; 31 of 38 had liver metastases, and 20 of 38 had two or more sites of involvement. Eight patients had grade 4 toxicities, including sepsis (nonneutropenic) (one), watery diarrhea (two), and granulocytopenia (six). Grade 3 neurologic toxicities were observed in two (5%) patients and included slurred speech and gait disturbance. Objective response was 42% (95% confidence interval [Cl], 27% to 58%), including one clinical complete responder and 14 partial responders. Among the responding patients, the median time to treatment failure was 8 months. Two patients remain on treatment at 10+ and 16+ months: median survival has not been reached. The results of this multiinstitutional trial suggest that the addition of IFN to 5FU enhances the objective response rates achieved in patients with advanced colorectal carcinoma and that the toxicities of this regimen are acceptable.