Thirty one patients with multiple ventricular septal defects were studied by cross sectional echocardiography, conventional pulsed and continuous wave Doppler, colour flow imaging, and left ventriculography to determine the relative diagnostic benefits and pitfalls of each technique. The patients studied had a wide range of congenital heart defects with 19 patients having isolated multiple ventricular septal defects, three with associated tetralogy of Fallot, five with double outlet right ventricle, three with complete transposition and ventricular septal defect, and one with a complete atrioventricular septal defect. In 23 patients the defects were inspected at operation. Cross sectional imaging with integrated pulsed and continuous wave Doppler correctly identified multiple defects in only 12 (39%) patients. In contrast, colour flow imaging was accurate in 24 (77%) patients and left ventriculography in 20 (65%) patients. When patients were subdivided on the basis of relative peak systolic ventricular pressures into restrictive defects (18 patients) and non-restrictive defects (13 patients) the diagnostic value of colour flow imaging was different for each group. Colour flow mapping correctly identified multiple ventricular septal defects in 16/18 (89%) patients with restrictive defects but only 8/13 (62%) with non-restrictive defects. The comparative diagnostic accuracy of left ventriculography was 15/18 (83%) in the restrictive group and 5/13 (38%) in the non-restrictive group. Colour flow imaging was the single investigative technique with the greatest diagnostic accuracy in the diagnosis of multiple ventricular septal defects. It failed to be consistently accurate in defined subgroups with non-restrictive defects as did left ventriculography. The greatest overall diagnostic accuracy in this series was obtained when both colour flow imaging and ventriculography techniques were used in combination in a complementary fashion.