We have previously identified, in intron 7 of the TFPI gene, a T to C single-base polymorphism (-33T-->C) which is strongly associated with total circulating TFPI antigen levels. Here we examined the influence of this polymorphism on the risk of venous thromboembolism. The polymorphism was identified in the PATHROS study population (330 cases with venous thromboembolism and 826 controls). The CC genotype was found in 6.4% of cases and 10.2% of controls (age-adjusted odds ratio 0.6; 95% CI 0.3-0.9; p = 0.03). This protective effect persisted after adjustment for oral contraception and the factor V Leiden and prothrombin gene polymorphisms. In 171 controls and 49 cases in whom blood was taken at least three months after the thrombotic event, the CC genotype was associated with significantly higher total TFPI levels than the TT genotype. These results suggest that the CC genotype of the TFPI intron 7 polymorphism is an independent protective factor for venous thromboembolism, an effect probably mediated by increased TFPI levels.