This study demonstrates a subject specific approach for simulating blood flow within a left ventricle (LV) by a combined analysis of computational fluid dynamics (CFD) and magnetic resonance imaging (MRI). Time dependent LV morphology and wall movements are measured by 3D multi-slice MRI. The information is then used to build numerical meshes for CFD simulation which provides detailed 3D LV velocity and pressure distributions in the LV cavity. Issues related to mesh generation and the prescription of valvular boundary conditions are also discussed. The predicted flow patterns are compared to that of the published MR measurements, demonstrating the effectiveness of the proposed approach.