Recently, we reported that serum concentration of IL-18 is strikingly high in patients with adult-onset Still's disease (AOSD). The aim of the present study was to screen for genetic polymorphisms in the human IL-18 (hIL-18) gene and to determine the association of polymorphisms with susceptibility to AOSD. We investigated the 6.7 kb region upstream of exon 2 of hIL-18 gene, in which a promoter activity had been reported. Sixteen AOSD patients, 144 rheumatoid arthritis (RA) patients and 92 healthy control individuals were studied. We found seven single nucleotide polymorphisms and a single 9 bp insertion which were frequently present in the AOSD patients. Three haplotypes including a unique combination of these polymorphisms were also determined. Of them, haplotype S01 contained all eight of these polymorphisms. The frequency of individuals carrying a diplotype configuration, ie a combination of two haplotypes, of S01/S01 was significantly higher in the AOSD patients than in the healthy controls (P=0.00059, Fischer's exact probability test, odds ratio [OR]=7.81, 95% confidence interval [95% CI]=2.48-24.65) and the RA patients (P=0.015, Fischer's exact probability test, OR=4.0, 95% CI=1.39-11.54). We therefore conclude that possession of the diplotype configuration of S01/S01 is a major genetic risk factor for susceptibility to AOSD.