Coagulation factor XIII is a transglutaminase catalysing the crosslinking of fibrin chains as well as the formation of covalent links between several extracellular matrix proteins such as fibronectin, vitronectin and collagen. By mediating the incorporation of alpha2 antiplasmin into the fibrin network, this factor also interferes with fibrinolysis. Increased plasma factor XIII activity was reported by our laboratory 30 years ago in hypertriglyceridemic subjects who also displayed increased activity of serum cholinesterase, a marker of hepatic protein synthesis, and a delayed diluted, blood clot lysis time. Recent data in the literature emphasize a relationship between insulin resistance (metabolic syndrome) and increased plasma levels of factor XIII, confirming our results. It was also reported that a faster activation of this factor related to the Val 34 leu polymorphism provides protective effect against myocardial infarction and stroke, this effect being however negated in patients with insulin resistance and high plasma levels of plasminogen activator inhibitor-1. The pathogenic role of factor XIII in atherothrombosis seems to be bivalent. On the one side, an increased activity would favor the persistence of fibrin depositions and increase plaque burden, while on the other side it would reduce plaque vulnerability and the risk of downstream embolization.