To explore the effects of ABO incompatibility between recipient and donor on HLA-matched nonmyeloablative allogeneic peripheral blood stem cell transplantation (NAST), a retrospective, cohort study was performed. Among 24 HLA-matched NAST, 15 were major ABO-incompatible and 9 minor. Control group included 24 HLA-matched NAST with ABO-compatible grafts. Nonmyeloablative conditioning regimens consisted of CTX, Ara-C and ATG. The patients were given cyclosporine A and mycophenolate mofetile for prophylaxis of acute GVHD. The ABO-incompatible patients received grafts depleted erythrocytes by hydroxyethyl starch (HES) sedimentation. The results showed that successful and stable engraftment was established in 23 patients. No recipient developed clinically immediate hemolysis during graft infusion, but 2 recipients experienced delayed hemolysis attributable to the ABO incompatibility. The median time of granulocyte counts >0.5 x 10(9)/L and platelet >30 x 10(9)/L was 11 and 14.9 days, respectively. In ABO major incompatible group, the onset of erythropoiesis after NAST was delayed. One out of 10 recipients with blood group "O" in this group developed pure red cell aplasia (PRCA), lasting 5 months. The acute GVHD occurred in 7 out of the 24 patients. The chronic GVHD occurred in 5 of 21 cases. Relapse was observed in 2 patients with acute leukemia. The actuarial probability of disease-free survival at 2 years was 63.3%. In conclusion, ABO-incompatible grafts for NAST have no adverse effect on engraftment, recovery of platelets, incidence of GVHD, relapse rate or survival. ABO-incompatible NAST is fairly safe if there is indication, however, the onset of erythropoiesis is delayed when major ABO mismatched.