Numerical analysis of the aortic valve has mainly been focused on the closing behaviour during thediastolic phase rather than the kinematic opening and closing behaviour during the systolic phaseof the cardiac cycle. Moreover, the uid-structure interaction in the aortic valve system is mostfrequently ignored in numerical modeling. The e ect of this interaction on the valve's behaviourduring systolic functioning is investigated. The large di erences in material properties of uid andstructure and the nite motion of the lea ets complicate blood-valve interaction modeling. This hasimpeded numerical analyses of valves operating under physiological conditions. A numerical method,known as the Lagrange multiplier based ctitious domain method, is used to describe the large lea etmotion within the comp! utational uid domain. This method is applied to a three-dimensional niteelement model of a stented aortic valve. The model provides both the mechanical behaviour of thevalve and the blood ow through it. Results show that during systole the lea ets of the stented valveappear to be moving with the uid in an essentially kinematical process governed by the uid motion.