In the light of reduced intensity conditioning regimens for allogeneic transplantation, monitoring of donor cell engraftment acquires new relevance. We analysed the clinical significance of haematopoietic chimaerism as a parameter of patient outcome and detection of relapse for early intervention by donor lymphocyte infusion (DLI) after allogeneic transplantation. Between July 1994 and March 1999, 101 adult patients with malignant disease were evaluated. Median follow-up was 15 months (range 0.7-56.5) after transplantation. Patients received busulphan-containing (n = 82) or total body irradiation (TBI)-containing (n = 19) regimens. Fifteen out of 98 (15%) patients with predictive chimaerism analyses relapsed, 5 out of 20 (25%) with mixed chimaerism (MC) and 10 out of 78 (13%) with complete donor chimaerism (CC) before apparent relapse. Seven patients received donor lymphocyte infusions (DLI) as relapse therapy with conversion from MC to CC in all three patients with successful DLI. Our data stress the importance of chimaerism monitoring after allogeneic transplantation and demonstrate the more frequent occurrence of disease relapse in patients showing MC, rather than CC, after transplantation. Moreover, the assessment of chimaerism has been shown to be a valuable tool in monitoring the efficiency of donor lymphocyte infusions for relapse.