In the past 10 years 201 HLA-identical bone marrow transplantations (BMT) were performed with major ABO incompatibility in 41 (20%) and minor ABO incompatibility in 35 (18%) patients. ABO compatibility between donor and recipient showed no influence on granulocyte and platelet recovery after BMT. Erythrocyte reconstitution was significantly (p < 0.01) delayed for about 1 week in major ABO-incompatible BMT. In addition, a pure red cell aplasia lasting for 2-5 months occurred in 6 out of 21 blood group 0 patients who received transplants of group A. The rate of graft rejection, incidence of graft-versus-host disease as well as the leukemic relapse rate were similar in ABO-compatible and ABO-incompatible BMT. The probability of a 10-year survival after BMT is independent of ABO compatibility between donor and recipient.