Recent studies have suggested that rituximab has clinical activity and modulates antiapoptotic proteins associated with drug resistance in chronic lymphocytic leukemia (CLL). We performed a randomized phase 2 study to determine the efficacy, safety, and optimal administration schedule of rituximab with fludarabine in previously untreated CLL patients. Patients were randomized to receive either 6 monthly courses of fludarabine concurrently with rituximab followed 2 months later by 4 weekly doses of rituximab for consolidation therapy or sequential fludarabine alone followed 2 months later by rituximab consolidation therapy. A total of 104 patients were randomized to the concurrent (n = 51) and sequential (n = 53) regimens. During the induction portion of treatment, patients receiving the concurrent regimen experienced more grade 3 or 4 neutropenia (74% versus 41%) and grade 3 or 4 infusion-related toxicity (20% versus 0%) as compared with the sequential arm. The consolidation rituximab therapy was tolerated well in both arms. All other toxicities were similar in the 2 arms. The overall response rate with the concurrent regimen was 90% (47% complete response [CR], 43% partial response [PR]; 95% confidence interval [Cl] 0.82-0.98) compared with 77% (28% CR, 49% PR; 95% Cl, 0.66-0.99) with the sequential regimen. With a median follow-up time of 23 months, the median response duration and survival have not been reached for either regimen. Rituximab administered concurrently with fludarabine in previously untreated CLL patients demonstrates marked clinical efficacy and acceptable toxicity. Phase 3 studies using this combination approach for patients with CLL are warranted.