Magnetic resonance images were obtained in six patients with known or suspected interatrial septal defects (ASD) and 33 subjects without congenital heart disease. Image planes were oriented to the long and short axes of the left ventricle and septum and provided reproducible display of pertinent anatomic landmarks. Of the images in 33 control subjects, three (9%) were falsely positive showing apparent ASD. ASDs were correctly identified and localized in all six patients (five secundum, one primum). Long-axis views provided excellent display of the location and diameter of defects. All were confirmed at cardiac catheterization and four at surgery with good agreement in measurement of defect size and location. All showed signs of right-sided volume overload. Five had previous echocardiography, which showed definite or suspected ASD in four.