Behcet's disease (BD) is a chronic inflammatory disorder of unknown cause, characterized by recurrent oral ulcerations, genital ulcerations, ocular and skin lesions. Although the exact pathogenesis for BD is not completely understood, it has been suggested that the disease is triggered in genetically susceptible individuals by environmental factors, such as microbial agents. It is noted that multiple genes, including MHC and non-MHC genes, are implicated in the pathogenesis of BD. Although the HLA-B51 is known to be the candidate gene showing the strongest association with BD, it is necessary to be determined whether this HLA molecule is directly involved in the pathogenesis of BD. Cross-reactivity between microbial 65-kD and human 60-kD heat shock proteins is demonstrated to cause an increased T cell (particularly gammadelta T cell) response. The resultant overexpression of pro-inflammatory cytokines (mainly Th-1 type) from several immune cells seems to be responsible for the enhanced inflammatory reaction, and this may be associated with the genetic factors.