Recent studies have shown elevated IL-10 levels in several rheumatic autoimmune diseases, and particularly in systemic lupus erythematosus (SLE). Such changes may have a genetic basis. We studied two novel polymorphic dinucleotide repeats in the IL-10 promoter region (IL10.G and IL10.R) in order to investigate their possible significance in association with this condition in a group of 56 Caucasian SLE patients compared with 102 ethnically matched controls. The results show that there is an allelic imbalance between SLE patients and controls at the IL10.G microsatellite; this observation is supported by a significant difference in genotype distribution. The nature of autoantibody production and the presence or absence of renal involvement also appeared to be associated with certain IL10.G microsatellite alleles, although the small size of individual clinical sub-groupings may have influenced this result. No association with the IL10.R microsatellite was observed. Overall, the differences observed at the IL10.G microsatellite between SLE patients and controls suggest that the IL-10 locus contributes to the genetic background important for the development of this disease. Although the moderate sample size described in this study requires that the results be interpreted carefully, they provide an interesting and useful framework for future study.