Susceptibility to primary sclerosing cholangitis (PSC) is associated with HLA-B8, -DR3, -DR2, and -DR6. It is not established whether these HLA genes or closely linked genes confer the primary disease susceptibility. MICA and MICB genes are found in the class I region between HLA-B and DRB. MICA is expressed in gastrointestinal epithelium and activates ␥␦ T cells in the gut. Because PSC is strongly associated with inflammatory bowel disease, we investigated whether MICA and MICB contribute to the HLA-associated genetic susceptibility to develop PSC. The study included 130 PSC patients and 306 healthy controls, previously typed for HLA class I and II genes, typed for 5 MICA and 15 MICB microsatellite alleles. The phenotype frequencies of MICA5.1 and MICB24 were significantly increased among PSC patients compared with controls (90% vs. 74%; odds ratio [OR] ‫؍‬ 3.2; P c ‫؍‬ 3 ؋ 10 ؊3 and 58% vs. 29%; OR ‫؍‬ 3.3; P c < 1 ؋ 10 ؊7 , respectively). When stratified for B8-or DR3-positive and -negative individuals, the association of these markers to PSC was no longer evident. However, we observed that B8 and DR3 were associated to PSC only in the presence of both MICA5.1 and MICB24 markers. The frequency of individuals carrying all 4 alleles was significantly increased among the PSC patients compared with controls (49% vs. 18%; OR ‫؍‬ 4.5; P c < 1 ؋ 10 ؊7 ). Our data indicate that PSC is associated to the extended B8-MICA5.1-MICB24-DR3 haplotype. (HEPATOLOGY 2001;34:625-630.)Primary sclerosing cholangitis (PSC) is a chronic liver disease characterized by inflammatory and fibrotic bile duct lesions forming multiple strictures and ectatic dilatations of the intra-and extrahepatic bile ducts.