The current study investigates the role of regulatory T (Treg) cells in the pathogenesis of ocular attack in patients with Behcet’s disease (BD). Nineteen BD patients with ocular complications (BDo), including 11 BD patients with ocular attack (BDa) and eight BD patients with inactive ocular complications (BDi), were studied. Four BD patients without ocular complications (BDwo) were also evaluated as controls. All patients were prospectively followed by our outpatient clinic between autumn 2004 and spring 2005. CD4+ CD25+^bright T cells (Treg cells) from peripheral blood were measured by flow cytometry. The percentages of Treg cells in CD4+ T cells from BDo were significantly decreased before ocular attack compared with those after ocular attack. Moreover, surprisingly, these levels before attack were significantly lower than normal level, whereas the percentages of Treg cells in both BDi and BDwo patients were normal. Treg cells were significantly decreased in BDa before active ocular attack. These findings suggest that Treg cells play an important role in ocular attack in BD patients. In addition, decreased percentages of Treg cells may be a predictive marker of ocular attack in BD patients allowing treatment of BD patients before an ocular attack.