OBJECTIVE To evaluate the effectiveness of adding interferon (IFN) alfa-2b to chemotherapy in the induction treatment of low-grade non-Hodgkin's lymphoma (NHL), and to assess the role of maintenance IFN. METHODS A multicenter, two-phase controlled trial with double randomization was conducted in 155 patients with low-grade NHL. In the first randomization, 78 patients received cyclophosphamide, vincristine, and prednisone (CVP) and IFN, 3 MU/m2 three times a week for 3 months, and 77 patients received CVP alone. Responding patients were randomized to receive IFN for 1 year versus observation. RESULTS Of 144 assessable patients, 73 received CVP + IFN and 71 received CVP. Responses were similar: CVP + IFN 79% versus CVP 76% (P = .62). The number of patients who did not complete the treatment was higher in the CVP + IFN group than in the CVP group (18% v 4%; P = .009), although the received dose-intensity of chemotherapy was comparable. Duration of response and progression-free survival (PFS) were significantly higher in the CVP + IFN group than in the CVP group (P = .0004). However, we observed no differences in overall survival (OS) (P = .30), with a median follow-up for the surviving patients of 3 years. Grade 3/4 granulocytopenia was the most frequent toxicity and was similar in both groups (33% v32%). Eighty-three (74%) of the 112 responding patients were randomized to maintenance IFN or observation. The duration of response was similar between 42 patients that received IFN compared with 41 control patients (P = .83), independently of treatment previously administered. CONCLUSIONS Adding IFN alfa-2b to induction CVP in low-grade NHL did not induce a higher response rate, but it significantly increased the duration of the responses. We found significant differences in PFS that favored the patients who received CVP + IFN, but not in OS. To date, no additional benefit has been seen from the administration of IFN for maintenance.