OBJECTIVE An increased incidence of allele 2 of the interleukin-1 receptor antagonist gene (IL1RN*2) in several inflammatory diseases has recently been reported. The aim of this study was to examine a variable number tandem repeat (VNTR) polymorphism of the IL1RN gene in patients with juvenile idiopathic arthritis (JIA). METHODS Findings in 185 Czech patients with JIA were compared with those in 168 Czech controls, 50 JIA patients and 52 controls of Turkish origin, and 79 controls from central England. VNTR polymorphism analysis of IL1RN was performed by polymerase chain reaction using 2 flanking primers to amplify an 86-bp tandem repeat region in intron 2. RESULTS The frequency and carriage rate of IL1RN*2 were significantly increased in Czech JIA patients compared with the Czech controls (frequency 27.6% versus 15.8%; carriage rate 44.3% versus 26.2%). Increased frequency and carriage rate of IL1RN*2 were found in 23.3% and 40.0% of Turkish JIA patients and in 17.3% and 34.6% of ethnically matched controls. The high representation of IL1RN*2 in 52.3% of the 22 patients with extended oligoarthritis, 31.3% of the 56 patients with enthesitis-related arthritis, and 42.9% of the 14 patients with other arthritis was particularly responsible for the increased frequency of IL1RN*2 in the Czech JIA patients. We found no association of IL1RN*2 with disease activity or severity parameters. CONCLUSION Inheritance of IL1RN*2 may contribute to genetic susceptibility in several forms of autoimmune diseases, including JIA. The IL1RN*2 allele may be useful as a prognostic indicator of the evolution of an extended oligoarticular course of JIA.