Aim: To investigate T-bet mRNA and protein expression on peripheral blood monanuclear cells (PBMC) in patients with Behcet's disease with active uveitis. Methods: Blood samples were taken from 24 patients with Behcet's disease who had active uveitis and 16 healthy individuals. PBMC were subjected to analysis of T-bet mRNA nd protein expression using semiquatitative reverse tran scriptase-polymerase chain reaction (RT-PCR) and western blot, respectively. The products from PCR were sequenced. In order to determine the influence of activation on T-bet expression, the phytohaemagglutinin (PHA) stimulated PBMC from each sample were also evaluated for expression of T-bet mRNA and protein. Results: A significantly increased T-bet mRNA accumulation was detected in the samples from patients with active Behcet's disease compared with that in controls. A 62 kDa band was detectable in patients with active Behcet's disease, but not in controls. No difference was found between patients with Behcet's disease who had active uveitis and normal controls concerning the expression of either T-bet mRNA or its protein after stimulation with PHA for 72 hours. Conclusion: Behcet's disease is associated with an upregulation of T-bet expression, which supports a role for the Thl subset of T cells in the pathogenesis of this disease.