Thrombin-activable fibrinolysis inhibitor (TAFI) is a recently described carboxypeptidase that is potentially involved in the regulation of fibrinolysis by decreasing plasminogen binding to the fibrin surface. This role makes the TAFI gene a good candidate in atherothrombotic diseases. The great interindividual variability of plasma TAFI antigen levels is poorly explained by lifestyle characteristics, thus suggesting its genetic determination. To test this hypothesis, the promoter and the 3'-untranslated region of the TAFI gene were screened for polymorphisms, and their contribution to the variability of plasma TAFI antigen levels was evaluated. Seven new polymorphisms are described, 5 in the promoter (C-2599G, -2345 2G/1G, A-1690G, G-1102T, and G-438A) and 2 in the 3'UTR (C+1542G and T+1583A). All these polymorphisms were in strong linkage disequilibrium with each other and with the previously described Ala147Thr polymorphism. They generated 4 main haplotypes, accounting for 80% of all observed haplotypes. In univariate analyses, all polymorphisms were associated with plasma TAFI Ag levels and, individually, contributed to a large fraction of plasma TAFI Ag levels, ranging from 20% to 52%. In a stepwise regression analysis including all polymorphisms, several combinations remained significantly and independently associated with plasma TAFI Ag levels: C+1542G associated with Ala147Thr, T+1583A, or -2345 2G/1G explaining 61.6%, 60.2%, and 58.1% of the variance, respectively. These findings clearly demonstrate that circulating levels of TAFI are strongly determined by polymorphic variations in the promoter and the 3'UTR of the TAFI gene. (Blood. 2001;97:2053-2058)