A 4-year-old girl with transfusion-dependent beta(0)-thalassaemia received an HLA-identical bone marrow transplant (BMT) from her beta(0)-thalassaemia trait sister. Prior to BMT, chromosomal analysis revealed the recipient to have 46,XX,9qh+, a polymorphic variant of the heterochromatin region of chromosome 9, which her donor did not have. Within 1 month post-BMT, 89% of nucleated bone marrow cells were of donor origin. One year later, donor engraftment had decreased to 44% and 34% in nucleated bone marrow cells and blood lymphocytes, respectively. By 2 years, donor lymphocyte engraftment fell to 5%, raising concern of possible graft rejection. To examine erythroid chimaerism, globin synthesis by individual erythroid progenitor cell derived colonies (BFU-E) was analysed. On days 1000 and 1130 post-BMT, 79% and 77% of colonies, respectively, synthesized beta-globin and therefore were of donor origin.