The aim of this study was to assess whether thymidylate synthase (TYMS) genotype, serum homocysteine, and folate concentrations were related to venous thrombosis in Behçet's disease (BD) patients. The study included 104 BD patients fulfilling the International Study Group Criteria for the diagnosis of BD and 121 healthy individuals-controls. Out of 104 patients, 50 (48%) had vascular involvement: 34 had active-history of venous thrombosis, 16 had arterial involvement (aneurysm), and 11 of these patients had both venous and arterial lesions as confirmed by Doppler ultrasound and/or angiography. Genotype analysis of the TYMS promoter enhancer region was determined by polymerase chain reaction. The distribution of the TYMS genotypes 2R/2R, 2R/3R, 3R/3R, 4R/2R, and 3R/3R were not significantly different between BD patients and control group (p>0.05; 16.5% vs 8.3%, 49.0% vs 53.9%, 31.7% vs 38.0%, 1.9% vs 0%, and 1.0% vs 0%, respectively). TYMS genotypes were not associated with thrombosis and serum homocysteine concentration in BD patients. The mean serum homocysteine level in patients with thrombosis (14.87+/-8.99 micromol/L) was significantly higher than the level in patients without thrombosis (10.78+/-3.81 micromol/L; p<0.05). Serum folate concentrations were not different between the BD patients and the healthy controls. The study results suggest that the distribution TYMS genotype in BD was not different from that of healthy controls. There was no relationship between TYMS genotype and the homocysteine levels in BD patients with thrombosis or without thrombosis.