We investigated the prevalence of the coagulation factor V gene G1691A mutation in 64 patients with Behcet's disease (BD) and in 107 apparently healthy individuals. The mutation was present in the heterozygous state in 37.5% of the patients with a history of deep vein thrombosis (12/32) and in 9.4% of the patients without any thrombotic event (3/32). Eleven healthy individuals were also heterozygous for the mutation (10.3%). The prevalence of the mutation in BD patients with and without thrombosis was significantly different (P = 0.0079). We conclude that the factor V gene mutation may play a major role in the development of venous thrombosis in BD.