Thirty-three patients with pulmonary hypertension (PH) and age-, sex-matched 15 controls were retrospectively evaluated. The pulmonary artery to ascending aortic ratio (PA-A ratio) measured by cardiac MRI was significantly higher in PH patients than that in controls. The patients with pulmonary arterial hypertension showed a significantly higher PA-A ratio compared to patients with other causes of PH. Increased PA-A ratio showed significant correlations with right ventricular (RV) dilatation and decreased RVEF. The PA-A ratio showed high sensitivity and specificity for detection of PH. The PA-A ratio using cardiac MRI is an easy surrogate marker for detection of RV dysfunction and PH.