Interleukin-7 (IL-7) is essential for T-cell development in the thymus and for the maintenance of peripheral T cells. IL-7 signals through IL-7R, that consists of the gammac-chain and an alpha-chain. Sequencing of IL-7Ralpha has revealed the existence of four single nucleotide polymorphisms (SNPs) (+510C/T, +1237 A/G, 2087T/C and +3110A/G), which all give rise to amino-acid substitutions. The aim of the present investigation was to evaluate the significance of IL-7Ralpha SNPs for the outcome in allogeneic stem cell transplantation (SCT). IL-7Ralpha polymorphisms were determined in 195 recipient and donor pairs from either matched sibling donors or matched unrelated donors (MUD). Genotyping of 173 normal controls was performed in parallel. In MUD transplants, the +1237 genotype of the donor was associated with survival after SCT, the mortality being highest and intermediate for the GG and AG genotypes, respectively (P = 0.023). This pattern was more pronounced with respect to treatment-related mortality (P = 0.003), while IL-7Ralpha genotypes were unrelated to the risk of relapse of leukaemia. The IL-7Ralpha +1237 genotype of the recipient and the genotypes of the other three polymorphisms, were not significantly associated with the outcome of SCT. These findings suggest that the IL-7Ralpha polymorphisms may be of importance for treatment-related mortality after SCT.