It is known that low-dose aspirin is effective in coronary artery therapy, although it has not yet been clarified how it exerts its action. Here, we report that treatment of coronary artery patients with 100 mg/day of aspirin does not attenuate thrombin generation, but reduces free thrombin by favouring the formation of thrombin/antithrombin (TAT) complexes. Antithrombin hyperactivation is mediated by inhibition of platelet factor 4 release from alpha-granules, leading to higher heparin availability.