Gated magnetic resonance imaging was performed to identify the morphologic characteristics of the ventricular septum in 17 normal volunteers and to localize a ventricular septal defect (VSD) in 22 patients using the multisectional spin-echo technique (0.35 tesla). Location of VSDs was corroborated by 2-dimensional echocardiography or angiography or both. The 4 portions of the ventricular septum could be easily identified, especially on transverse images. The VSD was visualized in 20 of the 22 patients. The smallest VSD identified was 0.3 cm in diameter. The 2 VSDs not identified were a small membranous VSD under conditions of excessive patient motion, causing poor-quality images, and a VSD in a 3-month-old infant in which a 3-mm gap existed between sections. The exact location of the VSD could be identified in the other 20 patients. The VSD was perimembranous in 10 patients, inlet in 4, complete atrioventricular canal type in 3 and outlet in 6. The outlet consisted of 1 subpulmonic and 5 malalignment defects. Transverse images were most effective for identification of VSD. Thus, magnetic resonance imaging is a useful noninvasive technique for identifying the ventricular septum and assessing VSDs. It has the potential to be an alternative to angiography in patients beyond infancy for definitive diagnosis and localization of VSD before surgery.