A 45-year-old woman with acute myelogenous leukemia developed platelet transfusion refractoriness (PTR) after the engraftment of an allogeneic peripheral blood stem cell transplantation (PBSCT) from her multiparous sister, which was attributed to HLA antibodies that could not be detected in the patient's serum before transplantation. She achieved neutrophil engraftment by day 18 and megakaryocytopoiesis and complete donor chimerism was confirmed in the bone marrow on day 21. IgG-class HLA antibodies were detected in her serum on day 24 after PBSCT; however, on day 15, no HLA antibodies were detected. The specificity of the antibodies that emerged in the patient closely resembled that of the antibodies found in the donor. The donor had probably been immunized during pregnancy by their partner's HLA-antigens expressed by the fetus. Consequently, transplanted donor-derived cells provoked HLA antibodies in the recipient early after PBSCT, and those HLA antibodies induced PTR. The presence of HLA antibodies should be examined at least in pregnant female donors whose recipients developed PTR attributable to HLA antibodies after SCT.