After organ transplantation, susceptibility to cancer is multifactorial, especially for skin carcinomas. Risk factors may include genetic susceptibilities, such as the control of cytokine production. Interleukin-10 is a cytokine that is implicated in tumorigenesis, and it has been shown that polymorphisms in its gene promoter correlate with differential amounts of production. The aim of this study was to investigate a possible association between interleukin-10 gene promoter polymorphisms and the occurrence of skin carcinomas after renal transplantation. Seventy kidney transplant recipients who developed a squamous cell carcinoma or a basal cell carcinoma were examined for polymorphisms in the interleukin-10 gene promoter using polymerase chain reaction based methods. Single base pair mutations were studied at positions -1082, -819, and -592. These patients were compared to 70 healthy controls and to 70 matched renal transplant recipients without cancer. The interleukin-10 secretion capability was tested in a subgroup of 40 of these patients by in vitro stimulation of peripheral mononuclear cells. Interleukin-10 genotypes and haplotypes were differently distributed in kidney transplant recipients who developed a skin carcinoma, but especially a squamous cell carcinoma, with an increased frequency of the GCC haplotype and a decreased frequency of the ATA haplotype. Subsequently, we found a shift in the predicted phenotypes from the low production phenotype to the high production phenotype. Secretion of interleukin-10 was strongly correlated to the production predicted phenotype, and tended to be higher in patients who developed a squamous cell carcinoma than in the others. These results indicate that interleukin-10 gene polymorphisms and interleukin-10 production capability may contribute to the development of skin squamous cell carcinomas after renal transplantation.