Behcet's disease (BD) is a chronic recurrent systemic vasculitis of unknown cause. Thrombotic tendency is another characteristics of BD. While hemorheological changes play crucial role in thrombosis, erythrocyte deformability (ED), which is one of the main determinants of microcirculation and blood viscosity, has not been studied before in BD. ED was measured by the filtration method in 13 active BD patients diagnosed according to the criteria proposed by International Study Group for BD (ISG-BD) and in 13 age-, sex-, weight-, smoking habit-, and blood pressure-matched healthy controls. ED was decreased in active BD patients compared to the healthy control subjects. The median deformability indices (DIs) in BD patients and healthy controls were 3.19 and 2.08 (p < 0.001), respectively. The decrease in ED may be one of the reasons for increased thrombotic tendency in BD. Alteration in ED possibly plays a crucial role in the etiopathogenesis of thrombotic complications in BD and thus may be a target for treatment. Drugs increasing ED, such as pentoxifylline, have already been shown to be beneficial in the treatment of BD. Therefore, we propose that the therapeutic mechanism underlying the beneficial effect of this drug in BD is likely to be the correction of impaired ED.