Phase II study of fluorouracil and recombinant interferon alfa-2a in previously untreated advanced colorectal carcinoma. We conducted a phase II clinical trial of fluorouracil (5FU) and recombinant interferon alfa-2a (rIFN alpha-2a) in 52 previously untreated patients with bidimensionally measurable metastatic colorectal cancer. During week 1, 5FU was administered as a continuous intravenous infusion, 750 mg/m2/d for 5 consecutive days. Intravenous bolus administration of 5FU 750 mg/m2 was given weekly for 7 weeks starting on day 12. rIFN alpha-2a (Roferon; Hoffman-LaRoche, Nutley, NJ), 9 x 10(6) U, was administered subcutaneously three times weekly during weeks 1 to 8. Patients were evaluated for response on week 9. Of 52 patients enrolled in the study, 51 were assessable for toxicity, and 45 were assessable for response. Fifteen patients experienced partial response, and one patient achieved a clinical complete response for an overall response rate of 35% (95% confidence interval [CI], 22%, 50%). Median duration of response is 7.5 months (range, 4 to 11 months). Seventy percent of patients entered on the study are alive with a median follow-up duration of 7 months. Twenty-five percent of patients developed grade 4 toxicity, and 82% developed grade 3 toxicity. One drug-related death in the presence of sepsis was reported, and two treatment-related seizures occurred. Our experience with this schedule produced a lower response rate with greater toxicity than previously reported. Current randomized trials comparing this schedule of 5FU with rIFN alpha-2a to 5FU plus folinic acid (leucovorin) or single-agent 5FU may determine its role in the treatment of advanced colorectal carcinomas. 