A three-dimensional, realistic model of an aortic mechanical heart valve and Valsalva sinuses was developed to predict, by means of a numerical time dependent simulation, the flow field during a fraction of the systolic period. The numeric simulation was performed upon a model of valve similar to a Carbomedics 27 mm placed in a physiologic aortic root shaped model, in which no symmetry planes were exploited to reach a more realistic level. Input data for the simulation have been acquired during an experimental session on the same valve, according to the guidelines of testing protocol for prosthetic heart valves. Flow was assumed to be Newtonian and laminar at low regime and the leaflets fixed in the fully open position. The forward flow of the systolic phase was investigated, and a comparison with experimental results was performed at peak systole, the most representative point of the cardiac cycle. The results of this simulation furnished a reasonable indication (in terms of fluid dynamics) parameters downstream of the prosthetic device, especially in Valsalva sinuses, the role of which is proven to affect the valve's performance.