Although right ventricular (RV) function is an important determinant of morbidity and mortality in patients with primary pulmonary hypertension (PPH), there have been no clinically validated quantification methods to date. The first derivative of RV pressure (dP/dt) is a good index of contractility, but it depends on preload. dP/dt divided by end-diastolic volume (EDV), that is, dP/dt/EDV, on the other hand, is an index of contractility relatively independent of preload. However, the measurement of accurate RV EDV is difficult because of RV complex geometry. Real-time three-dimensional (3D) echocardiography allows us to measure ventricular volume irrespective of its shape. To investigate the clinical feasibility and significance of 3D echocardiography in evaluating RV function in patients with PPH by measuring RV EDV and dP/dt/EDV, 13 patients with PPH (41+/-20 years, four men) underwent echocardiography, a 6-min walk distance (mWD) test and blood sampling within 1 week of invasive hemodynamic measurements. RV dP/dt was estimated from a continuous wave Doppler-determined tricuspid regurgitant velocity. RV EDV was measured by both two-dimensional (2D) biplane Simpson method (EDV(2D)) and real-time 3D echocardiography (EDV(3D)). RV dP/dt/EDV was calculated using EDV(2D) and EDV(3D). EDV(3D) showed better correlations than EDV(2D) with the invasive and non-invasive parameters of RV function, suggesting the validity of volume measurement by 3D echocardiography. RV dP/dt/EDV(3D) correlated well with disease severity, whereas dP/dt and dP/dt/EDV(2D) did not. In patients with PPH, 3D-echocardiography-determined RV dP/dt/EDV and EDV seem to be potential markers of disease severity.