Interleukin-10 (IL-10) has a critical role in the regulation of immune responses. The relative contribution of genetic and environmental factors to IL-10 production is under debate. We performed a twin study in 246 monozygotic and dizygotic twins to assess the heritability of IL-10 production after LPS stimulation in whole blood. In addition, the influence of promoter single nucleotide polymorphisms (À1082, À819 and À592) on transcriptional activity and their binding to nuclear factors was studied in luciferase reporter gene and electrophoretic mobility shift assays. IL-10 production showed a genetic determination with a heritability of 0.5. Decreasing body mass index (BMI), smoking and female gender lead to decreased IL-10 production. In monocytes, the À1082A allele showed higher binding affinity to the transcription factor PU.1 resulting in decreased transcriptional activity of À1082A promoter haplotypes. Genetic determination of IL-10 secretion is probably lower than that previously reported. Fifty percent of the observed variability explained by genetic factors. Female individuals produce less IL-10 than male subjects. Environmental factors like smoking and decreasing BMI exert suppressing effects on IL-10 production. Although the À1082A allele shows higher binding affinity to the PU.1 transcription factor and lower transcriptional activity, this polymorphism probably explains only a small fraction of the observed heritability.