Blood group incompatibility between donor and recipient of allogeneic stem cell transplants may be associated with post-transplant erythroid aplasia. A total of 548 patients (pts) received allogeneic transplant for malignant and non-malignant hematologic disorders. In a retrospective analysis, the prevalence and outcome of pure red-cell aplasia (PRCA) in 44 pts with major and bi-directional ABO-mismatch were investigated. Bone marrow grafts were major ABO incompatible in 30 pts; there was bi-directional mismatch in the remaining 14 pts. The median number of transplanted mononuclear cells (NC) was 4.74 x 10(8)/kg (range 0.1-26.4) including CD34+ cells, 3.02 x 10(6)/kg (range 0.9-21.7). Granulocyte engraftment >0.5 x 10e9/l occurred after a median of 21 days (7-32), and platelet exceeded >50 x 10e9/l after a median of 23.5 days (12-109). Acute and chronic graft vs host disease (GVHD) developed in 23 (52%) and 26 (59%) of the patients, respectively. Six (13%) patients transplanted with major and bi-directional ABO-incompatibility developed PRCA. The treatment of PRCA consisted of plasmapheresis (PEX), rapid cyclosporine (CsA) discontinuation, donor lymphocyte infusions (DLI), erythropoietin (EPO), azathioprine, and rituximab. The therapy resulted in erythroid recovery in five out of six patients after a median of 13 months (range 3-16). The median number of transfused red blood cells (RBCs) was 36 U (range 8-57). With a median follow-up of 37 months, the 5-year probability of overall survival (OS) for the PRCA group was 66%. Major ABO mismatch may lead to delayed donor erythroid engraftment. It results in long-term transfusion dependence and, therefore, the risk of iron overload. The therapy is long lasting, but usually effective in majority of patients.