Background: Currently, there are no data on the association between right ventricular (RV) structure and ECG changes specific for idiopathic pulmonary arterial hypertension (IPAH). Therefore, we aimed to assess the accuracy of the recommended ECG criteria for predicting RV hypertrophy (RVH) and dilation in patients with IPAH. Methods and Results: Twelve-lead ECG and cardiovascular magnetic resonance imaging (CMR) were performed in 23 consecutive patients with IPAH aged 49.8±16.3 years. ECG criteria were referred to RV mass index and RV end-diastolic volume index as measured by CMR. Only the ECG voltage criteria based on R wave amplitude in lead V1, R wave amplitude in aVR, P wave amplitude in II and ventricular activation time in V1 were useful for differentiating between patients with and without RVH. A ventricular activation time in lead V1 of <0.01s excluded RVH, whereas R in V1 >6mm, R:S in V1 >1, R in aVR >4mm, R:S in V5 to R:S in V1 <0.04 and P in II >2.5mm confirmed the diagnosis. Only the ventricular activation time in V1 correlated with RV dilation and when >0.045s confirmed its diagnosis. Conclusions: Only a few of the recommended ECG criteria proved to be useful in the diagnosis of RVH or RV dilation in patients with IPAH. Changes in the cut-off values improved their accuracy.  (Circ J 2012; 76: 1744–1749)