Red blood cell (RBC) and platelet transfusion requirements in patients given nonmyeloablative versus conventional peripheral blood stem cell (PBSC) transplants from HLA-matched siblings were compared. Between December 1997 and March 2000, 40 patients, aged 21 to 67 years (median 51), with hematologic malignancies underwent nonmyeloablative allografts after either 2 Gy total body irradiation alone (n = 30) or 2 Gy total body irradiation preceded by fludarabine 30 mg/m(2)/d on days -4, -3, and -2 (n = 10). All received postgrafting mycophenolate mofetil and cyclosporine. Controls included 67 concurrent patients, aged 23 to 66 years (median, 46 years), given conventional PBSC transplants following high-dose conditioning and postgrafting methotrexate and cyclosporine. Among patients given nonmyeloablative transplants, 23% required platelet transfusions compared with 100% among patients given conventional grafts (P <.0001). Further, the number of platelet units given to nonmyeloablative recipients was reduced, with a median of 0 (range, 0 to 214) compared with a median of 24 (range, 4 to 358) after conventional transplantation (P <.0001). Sixty-three percent of nonmyeloablative recipients required RBC transfusions compared with 96% of those with conventional grafts (P =.0001). The number of RBC units transfused was also reduced, with a median of 2 (range, 0 to 50) compared with 6 (range, 0 to 34) after conventional transplantation (P =.0001). High transfusion requirements before transplantation and donor-recipient ABO incompatibility increased transfusion requirements in both patient groups, though neither significantly influenced the outcome of the analysis. Neither patient age, splenomegaly at transplantation, development of graft-versus-host disease, nor posttransplantation cytomegalovirus antigenemia or cytomegalovirus disease had statistically significant influences on posttransplantation transfusions.