BehÇet's disease is recognized as a systemic inflammatory disease of unknown etiology. The disease has a chronic course with periodic exacerbations and progressive deterioration. Previous reports have shown at least three major pathophysiologic changes in BehÇet's disease; excessive functions of neutrophils, vasculitis with endothelial injuries, and autoimmune responses. Many reports suggested that immunological abnormalities and neutrophil hyperfunction may be involved in the etiology and the pathophysiology of this disease. HLA-B51 molecules by themselves may be responsible, in part, for neutrophil hyperfunction in BehÇet's disease. T cells in this disease proliferated vigorously in response to a specific peptide of human heat shock protein (hsp) 60 in an antigen-specific fashion. T cells reactive with self-peptides produced Th1-like proinflammatory and/or inflammatory cytokines. This leads to tissue injury, possibly via delayed-type hypersensitivity reaction, macrophage activation, and activation and/or recruitment of neutrophils. These data shed new light on the autoimmune nature of BehÇet's disease; molecular mimicry mechanisms may induce and/or exacerbate BehÇet's disease by bacterial antigens that have activated T cells which are reactive with self-peptide(s) of hsp. This would lead to positive selection of autoreactive T cells in this disease.