OBJECTIVE Several recent studies have shown that the MHC class III region, located telomeric to HLA-DRB1, contains an additional genetic factor that predisposes to rheumatoid arthritis (RA). In this study, we investigate whether inhibitor of kappaB-like (IkappaBL), MICB or MICA located in the MHC class III region are the second susceptibility gene associated with RA. METHODS A total of 154 healthy controls and 140 RA patients were genotyped for HLA-DRB1, MICA, MICB and the polymorphism -62 of the IkappaBL gene. RESULTS A significant increase of HLA-DRB1 shared epitope (SE) alleles was detected in RA patients (61.4 vs 43.5%, P(c) = 0.01, OR = 2.1, 95% CI = 1.3-3.3). Among SE alleles, the HLA-DRB1*0401 (13.5 vs 5.1%, P(c) = 0.04, OR = 3.2, 95% CI = 1.3-8.1) and HLA-DRB1*0404 (6.4 vs 1.2%, P = 0.02, P(c) = NS) showed the most significantly association with RA. No increase of risk was associated with HLA-DRB1*01. Remarkably, the allele MICB*004 was also significantly associated with RA susceptibility (40.7 vs 23.3%, P(c) = 0.01, OR = 2.2, 95% CI = 1.3-3.7). MICB*004 was in linkage disequilibrium with HLA-DRB1*0404 (lambda(s) = 0.33) and HLA-DRB1*0405 (lambda(s) = 0.34). However, MICB*004 was also increased in HLA-DRB1 SE negative patients (37 vs 21.5%, P = 0.04). No significant association between IkappaBL and MICA with RA was found. CONCLUSIONS MICB*004 allele was associated with RA susceptibility. This allele was in linkage disequilibrium with HLA-DRB1*0404 and DRB1*0405. The association of MICB with RA susceptibility and the functional role of MIC genes in the pathogenesis of RA converts MICB into a candidate to be an additional MHC gene associated with RA susceptibility.