Left ventricular (LV) diastolic dysfunction often precedes systolic dysfunction. Thus, the assessment of LV diastolic function is imperative for the early diagnosis of the disease, and many indices have been developed. Most, however, exclude pericardial and right ventricular (RV) effects, both of which alter LV pressure and volume. This study used a previously developed model of the cardiovascular system to examine how the pericardium and the RV affect these indices. By this approach, we could set the intrinsic LV compliance. We compared this to the LV diastolic indices derived from LV pressure and volume measurements. The analysis shows that RV and pericardial pressures affect the measured LV pressures and volumes and hence, the indices of LV diastolic function. Even with completely accurate measurements for pressure and volume, as in the simulation, ignoring the RV and the pericardium resulted in errors ranging from −38% to +37% in derived indices of LV function. Further, ignoring ventricular interdependence minimizes the ability of the indices to separate normal from disease states. To improve accuracy, RV and pericardial effects must be considered when calculating LV diastolic indices.