Cancer and Leukemia Group B conducted a phase II study to evaluate the safety and efficacy of a reduced-intensity conditioning regimen with allogeneic transplantation to treat patients with recurrent low-grade B cell malignancies. Patients over age 18 with a diagnosis of relapsed, chemotherapy-sensitive disease underwent transplantation with a matched sibling donor, and conditioning with cyclophosphamide (1 g/m²/day × 3) and fludarabine phosphate (25 mg/m²/day × 5). Graft-versus-host prophylaxis included cyclosporine or tacrolimus plus low-dose methotrexate. Forty-four evaluable patients with a median age of 53 and median of 2 prior regimens were accrued. Sixteen patients had follicular non-Hodgkin lymphoma and 28 had histologies including 7 indolent B cell lymphomas, 4 mantle cell, 15 chronic lymphocytic leukemia (CLL), and 2 prolymphocytic leukemia (PLL) patients. The 6-month treatment-related mortality (TRM) was 2.4% and 3-year TRM was 9%. Three-year event-free and overall survival were 0.75 and 0.81 for the follicular patients, 0.59 and 0.71 for the CLL/PLL patients, and 0.55 and 0.64 for the other histologies. The incidence of grade II-IV acute graft-versus-host disease (GVHD) was 29%, and extensive chronic GVHD was 18%. This report demonstrates that allogeneic sibling transplantation with a reduced-intensity conditioning regimen is safe and efficacious for patients with advanced indolent B cell malignancies enrolled on a Cooperative Group study.