The polymorphic gene expression of CYP2C19 causes individual variability in drug metabolism and thereby in pharmacologic and toxicologic responses. We genotyped 286 patients and their donors for the CYP2C19 gene who underwent allogeneic transplantation for various diseases and analyzed their outcome. Patients were classified as: poor metabolizers (PMs; 3.1%), intermediate metabolizers (IMs; 24.5%) and extensive metabolizers (EMs; 72.5%). Patients genotyped as PMs had significant higher hepato- and nephrotoxicities compared to IMs or EMs. Maximum bilirubin and serum creatinine levels measured after transplant were approximately twofold higher than those of EMs or IMs. The increased toxicity resulted in an increased 4-year estimate for transplant-related mortality (TRM) with 50+/-18.6% for PMs compared to 25.1+/-3.7% for EMs (P<0.018) and 22.7 +/-5.6% for IMs (P<0.042), whereas no significant influence for relapse rate, overall survival or incidence of acute graft-versus-host disease grade 2-4 were found between the groups. Multivariate analysis including all potential factors that might influence TRM confirmed that the genotype of CYP2C19 is an independent factor, which influenced TRM significantly. These results suggest that genotyping for CYP450 2C19 can help to identify patients with higher risk for TRM.