The ratio of early mitral inflow to early mitral annular velocity (E/E') was suggested as a useful index in the estimation of left ventricular filling pressure. This study was performed to validate the clinical usefulness of E/E' ratio in a large number of patients. Simultaneous left ventricular pressure measurements and Doppler examinations were performed in 200 consecutive patients at the cardiac catheterization laboratory. The E/E' ratio correlated well with pre-A-wave pressure (r = 0.74, P <.001), and the correlation was not dependent on the left ventricular systolic function (ejection fraction [EF] > or = 50%: r = 0.74, P <.001 versus EF <50%: r = 0.70, P <.001). The E/E' ratio of > or = 9 best discriminated elevated (<12 mm Hg) from normal left ventricular pre-A pressure with a sensitivity of 81% and a specificity of 80%. Our study results also showed that quantitative estimation of left ventricular pre-A pressure could be suggested by the simplified equation of pre-A pressure = E/E' + 4 with reasonable accuracy.