Assessment of donor chimerism is becoming increasingly important in patients undergoing reduced-intensity conditioning (RIC) allogeneic bone marrow transplants, due to the possibility of mixed chimeras. This regimen has been used successfully for patients with leukemia and genetic disorders with donor chimerism occurring in the myeloid, lymphoid, and/or erythroid lineages. Less toxic RIC expands the potential application of stem cell transplants to patients with nonmalignant disorders of hematopoiesis, such as the severe form of Glanzmann thrombasthenia, who previously were not considered suitable candidates based on risk-benefit analysis. To assess megakaryocyte/platelet chimerism after stem cell transplantation conducted with RIC, we used restriction fragment length polymorphism (RFLP) and sequence analyses of the HPA-3 polymorphism in the megakaryocyte/platelet-specific glycoprotein αIIb. In this study we show that at 23 weeks post-RIC, a leukemia patient acquired the HPA-3 donor phenotype at the DNA and platelet RNA levels.