Data on 477 patients with hematologic malignancies undergoing allogeneic bone marrow transplantation from HLA-identical siblings were analyzed for correlation between donor-recipient ABO blood group incompatibility and the development of elevated bilirubin levels (over 17 mmol/l) after transplantation. The median bilirubin on day 15 after transplant and the maximum bilirubin in the first 100 days were significantly higher in 155 patients with ABO-mismatched donors compared with 322 patients with ABO-matched donors. In univariate analysis, age > 16 years (P = 0.000006), ABO incompatibility (P = 0.0004), a conditioning regimen other than cyclophosphamide-total body irradiation (P = 0.0005) and a diagnosis other than acute leukemia (P = 0.01) were associated with a higher probability of developing elevated bilirubin. Incidence of clinically diagnosed graft-versus-host disease (GVHD), and transplant-related mortality, relapse rates and overall survival were not influenced by ABO incompatibility. The hyperbilirubinemia was therefore unlikely to be the result of an increased incidence of hepatic complications such as GVHD or veno-occlusive disease. We suggest that studies on serious transplant-related complications such as GVHD and veno-occlusive disease which rely on bilirubin values for diagnosis should take donor-recipient ABO incompatibility into account.