We have studied long-term engraftment in 24 multiply transfused patients transplanted for severe aplastic anaemia (SAA) 2-7 years previously from HLA identical sibling donors. All 24 patients had engrafted initially; nine (38%) developed grade II-IV a-GVHD, but only 5 (21%) developed chronic GVHD, which was mild, localized and transient. In 22 cases DNA 'fingerprint' analysis using a hypervariable minisatellite DNA probe (33.15) confirmed the donor/recipient origin of patient peripheral blood (PB) nucleated cells. Red cell antigens and PB lymphocyte chromosomes were also analysed in informative cases. In 19 patients (79%) PB cells were of donor origin confirming sustained engraftment, whereas five (21%) had PB cells of recipient origin. In four of these five cases complete autologous reconstitution was demonstrated. In one case DNA fingerprinting revealed mixed haemopoietic chimaerism. In three of the four cases of autologous reconstitution there had been a previous episode of late graft failure. The low incidence of chronic GVHD in the study group was not explained by autologous reconstitution or mixed chimaerism. We conclude that the hypervariable minisatellite probes are valuable in the study of engraftment after BMT, especially when patient and donor are HLA identical, of the same sex, and have the same ABO-Rh blood type. Pre-transplant specimens from the patient are not necessary for interpretation of the results provided that DNA from the donor is available.