Twenty-six children who underwent an arterial switch operation for transposition of the great arteries were examined with MRI and Doppler ultrasound (US). The aim was to determine the capability of MRI for detecting a possible stenosis of the pulmonary arteries. In 10 children angiocardiography (ACG) was done after the MRI examination. Using ACG as the gold standard, MRI was sensitive in the detection of pulmonary artery stenosis. MRI was more precise in the localization of a stenosis than was Doppler US. Right ventricular hypertrophy was a sensitive indicator of pulmonary artery stenosis. These results suggest that MRI could serve as a screening examination for pulmonary artery stenosis after an arterial switch operation.