The direct measurement of mean left atrial pressure is difficult and the ability to assess mean left atrial pressure from the left ventricular (LV) pressure tracing obtained at cardiac catheterization is clinically useful. The current study describes the limitations of using LV pre-a-wave pressure or LV end-diastolic pressure to estimate mean left atrial pressure and describe another index, mean LV diastolic pressure, which provides a better assessment of mean left atrial pressure.