Objective Although the etiology of Behcet's disease is not clarified yet, the vascular endothelial damage and thrombosis are prominent features of the disease. The underlying mechanisms of increased risk of thrombosis in Behcet's disease are not completely understood. It is stated that the changes in blood rheology such as erythrocyte deformability take part in the thrombosis. There are limited numbers of studies with conflicting results about the erythrocyte deformability in Behcet's disease. The previous studies were not limited to the untreated patients with active disease.