Magnetic resonance imaging (MRI) is considered the clinical reference standard for measuring the right ventricular (RV) volume and ejection fraction, although real-time 3-dimensional echocardiography (RT3DE) would be a preferred method owing to its convenience and availability for repetitive examinations. However, the feasibility, accuracy, and reproducibility of RT3DE have not been fully examined. The present study sought to validate the correlation of RT3DE with a 2-dimensional summation method compared to MRI for assessing the function of the right ventricle and to evaluate the RV function in patients with pulmonary hypertension (PH). Thirty patients with PH underwent both RT3DE and MRI. The right ventricle was reconstructed with RT3DE using a 2-dimensional summation method to analyze the MRI measurements. The RV end-diastolic volume, RV end-systolic volume, and RV ejection fraction were measured. Fifteen normal subjects underwent the same echocardiographic protocol for comparison. The RV end-diastolic volume index, RV end-systolic volume index, and RV ejection fraction measured using RT3DE correlated well with those measured using MRI (R = 0.96, p <0.001; R = 0.96, p <0.001; p = 0.93, and p <0.001, respectively). All inter- and intraobserver variability values for the RV end-diastolic volume, RV end-systolic volume, and RV ejection fraction were <17%. Both the RV end-diastolic volume index and the RV end-systolic volume index were significantly enlarged in those with PH compared to those in the normal subjects (RVEDVI 123 ± 42 ml/m² vs 74 ± 12 ml/m²; RVESVI 86 ± 33 ml/m² vs 26 ± 5 ml/m² in those with PH and the normal subjects, respectively, p <0.0001). In contrast, the RV ejection fraction was significantly reduced in the patients with PH compared to that in the normal subjects (30 ± 12% vs 65 ± 6%, respectively, p <0.01). Thus, RT3DE with a 2-dimensional summation method might provide comparable and feasible measurements of the RV volume in patients with PH compared to MRI.