We report a case of mild, clinically asymptomatic, immune thrombocytopenia after allogenic bone marrow transplantation (BMT) for chronic myeloid leukemia (CML) caused by the presence of a recipient- origin Br(a) antibody that recognized the donor platelets. Although the antibody titer decreased, it remained detectable more than 3 years after BMT. Chimerism studies were performed combining cytogenetics, blood cell phenotype studies, and genomic amplification of hypervariable sequences. Cytogenetic studies and molecular analysis of peripheral blood cells, purified B- and T-lymphocyte subpopulations, and bone marrow colonies showed the hematopoiesis to be of donor origin, but absorption-elution experiments with peripheral RBCs showed a small amount of recipient RBCs. The CML chimeric transcript was also detected by means of polymerase chain reaction on samples collected until day +867 post-BMT. This case shows that recipient-origin platelet alloantibodies can cause thrombocytopenia after BMT and that the persistence of small numbers of recipient cells (even leukemic) is not necessarily associated with hematologic relapse.