Multidimensional flow cytometry found an abnormal lack of CD16 expression detected on neutrophils after hematopoietic stem cell transplantation (HSCT) in bone marrow specimens from 2 patients. Chimerism studies were used to determine that the CD16– myeloid cells were donor in origin, consistent with a normal polymorphic variant, most likely a previously described Fcγreceptor IIIB gene deletion. Identical phenotypes were recognized in peripheral blood samples obtained subsequently from the respective donors. These donor-specific phenotypic abnormalities were transferred to patients who underwent successful HSCT. The distinction of these phenotypic abnormalities from neoplastic changes such as secondary myelodysplasias or relapse of original disease is important in posttransplantation monitoring of hematopoietic diseases.