Patients receiving an ABO-incompatible hematopoietic stem cell transplantation (SCT) are at an increased risk for immune-mediated hematological complications including immediate and delayed hemolysis, late red blood cell engraftment, and pure red cell aplasia. Much research effort has been invested in the unraveling of the immunological mechanisms underyling these complications and approaches to prevent them. Only minimal attention has been paid to the fact that in some SCT patients, even after years, a persistent patient A- and/or B-antigen is detected in the clinical laboratory, despite 100% white cell donor chimerism. The impact for the patient can be substantial: fear that the transplantation was not successful, concern of relapse, and other anxieties influence the quality of life. Little is known about the possible causes of this phenomenon, making appropriate counseling and reassurance of patients by the clinician difficult. In this letter, we describe two cases and a short review on the putative causes of persistent blood group antigens after SCT.