Interleukin-10 (IL-10) is an anti-inflammatory cytokine. Its production in humans is under genetic control, and genotype defines high or low producers of this cytokine. This study addresses the hypothesis that idiopathic inflammatory bowel disease (IBD) patients are more likely to have the low IL-10 producer genotype and phenotype. DNA was extracted from blood cells of patients with Crohn's disease (CD) or with ulcerative colitis (UC) for IL-10 genotyping. The frequency of the high IL-10 producer allele (-1082*G) was decreased in the whole IBD group (41% vs. 51%, P = 0.03) and in the UC patients compared with normal controls (37% vs. 51%; P = 0.04). Hence, there appears to be an association between the IL-10 genotypes and IBD. This suggests that individuals genetically predisposed to produce less IL-10 are at a higher risk of developing IBD, in particular, UC.