Certain cytokine gene polymorphisms have been shown to correlate with outcome of human leukocyte antigen (HLA) identical sibling donor stem cell transplantation (SCT), but in unrelated donor SCT such information is scarce. We have studied the association between cytokine gene polymorphism and transplant-related mortality (TRM) in 182 unrelated SCTs performed at a single center. We found association of polymorphism in the tumor necrosis factor alpha (TNFα) and interleukin-10 (IL-10) gene and TRM. Both the TNFd4 allele and the TNFα -1031C alleles are associated with high risk for TRM. Statistical analysis showed that both polymorphisms were present on a single haplotype. This haplotype was associated with high risk of TRM when present in recipient or donor, 55% (43%-67%) compared with 21% (12%-30%) when absent from both (P < .01). A further allele associated with this haplotype, TNFa5, is also associated with increased risk of TRM. For IL-10, presence of the donor R2-G-C-C haplotype was associated with decreased risk of TRM, 61% (43%-79%) versus 34% (25%-43%), P = .01. In contrast, possession of the R3-G-C-C haplotype by the donor predicted reduced risk of TRM, 30% (19%-41%, 95% Cl) versus 53% (40%-66%, 95% Cl), P= .01. No independent associations of cytokine polymorphisms with acute graft-versus-host disease were shown.