OBJECTIVE: Fibronectin (FN) plays an important role in the formation of stable arterial thrombi at the site of vascular injury. FN containing Extra Domain A (EDA+ FN) is absent from normal plasma, but elevated plasma levels of EDA+ FN are found in several pathological conditions. We hypothesized that EDA+ FN plays a special role in thrombosis. METHODS AND RESULTS: We used mouse strains constitutively including (EDA+/+) or excluding (EDA-/-) the EDA domain in all tissues and plasma. Using a flow chamber and the ferric-chloride injury model we found that EDA+ FN accelerates thrombosis both in vitro and in vivo at arterial shear rates. In EDA+/+ mice thrombi (>30 microm) grew faster when compared with EDA(WT/WT) (6.6+/-0.2 minutes versus 8.3+/-0.6 minutes, P<0.05) and the mean vessel occlusion time was shorter (9.9+/-0.4 minutes versus 14.6+/-1.7 minutes, P<0.05). However, the presence of EDA+ FN affected neither single platelet adhesion to subendothelium nor thrombosis in veins. In addition, the mortality rate of EDA+/+ mice after collagen/epinephrine infusion was twice that of EDA(WT/WT) or EDA-/- mice. CONCLUSIONS: Our findings reveal that EDA+ FN has prothrombotic activity, and its presence in plasma may worsen pathological conditions in which this form is elevated.