Perimembranous ventricular septal defects (VSD) have a close anatomical relationship to the aortic and tricuspid valve which may make percutaneous closure difficult. The authors' objective was to study the anatomy of perimembranous VSD by real-time 3D echocardiography. Fourteen patients aged 1 day to 3 years (mean 3 months) underwent transthoracic 2D and 3D echocardiography. Two patients (> 10 kg) underwent closure of the perimembranous VSD with an Amplatzer occluder. The other 12 patients were operated (surgical closure in 11 and circling suture with resorbable thread in 1 patient). The 3D echocardiography showed variations in the geometry of the perimembranous VSD (eccentric, oval, cleft). The maximal diameter of the septal defect by 3D echocardiography was on average 2 mm greater than the diameter by 2D echocardiography. The 3D echo enabled visualisation of the close relationship of the septal tricuspid leaflet, the aortic valve and the perimembranous VSD, showing the mechanism of its closure and the associated aortic regurgitant lesions. The profile of the prosthesis implanted percutaneously was well shown in the different 3D views. The authors conclude that real-time 3D echocardiography is a technique which allows accurate anatomic description of perimembranous VSD. As a complement to classical 2D methods, it gives a better appreciation of the maximal diameter of the septal defect and of its relationship to the aortic and tricuspid valves.