Objective: To analyze the effect of primary pulmonary hypertension (PPH) on cardiac function using MRI. Methods: In 12 patients (9 women; age range, 30 to 56 years), the diagnosis of PPH had been established by catheterization (mean ± SD pulmonary artery pressure [PAP] was 56 ± 8 mm Hg). With breath-hold cine MRI, a series of short-axis images was acquired covering the whole left ventricle (LV) and right ventricle (RV). The curvature, defined as 1 divided by the radius of curvature in centimeters, was calculated for the septum and the LV free wall in early diastole. Leftward ventricular septal bowing (LVSB) is denoted by a negative curvature. For the LV and the RV, the end-diastolic volume (EDV), stroke volume (SV), and volumetric filling rate were calculated. The control subjects were all healthy (n = 14; 11 women; age range, 20 to 57 years). Results: In the patients, LVSB was quantified in early diastole by the septal curvature of - 0.14 ± 0.07 cm-1, and the septal to free-wall curvature ratio of - 0.42 ± 0.21. LV EDV and LV SV correlated negatively with diastolic PAP (p = 0.004 and p = 0.04, respectively). In patients vs control subjects, RV SV was reduced (52 ± 12 mL vs 82 ± 11 mL, p < 0.0001); LV peak filling rate was smaller (2.2 ± 0.7 EDV/s vs 3.3 ± 0.5 EDV/s, p < 0.001); LV EDV was smaller (81 ± 23 mL vs 117 ± 19 mL, p = 0.001); and LV SV was smaller (49 ± 18 mL vs 83 ± 13 mL, p < 0.0001). Conclusion: In PPH, RV pressure overload leads to LVSB and reduced RV output. By decreased blood delivery, LV filling is reduced, which results in decreased LV SV by the Frank-Starling mechanism.