To gain a better understanding of the anatomy of doubly committed subarterial defects and its relation to findings at cross sectional echocardiography and angiocardiography, eight necropsy specimens from patients with this condition were examined, and preoperative echocardiograms and angiocardiograms from 313 patients with surgically or necropsy confirmed outlet defect were reviewed. Of these, 48 had doubly committed subarterial defects. Morphological review showed that doubly committed defects are roofed by the arterial valves in fibrous continuity because of lack of both the outlet septum and the "septal" aspect of the subpulmonary infundibulum. Angiocardiography had a lower sensitivity (50%) than echocardiography (95%) for diagnosis of doubly committed defects, but each was highly specific. In five (14%) of 35 available echocardiograms the arterial valves were normally offset, but in the remainder they were at the same level. The ventriculoarterial connection was concordant in 37/48 (77%), discordant in five (10%) of 48, and double outlet right ventricle in six (13%) of 48. Displacement of the fibrous raphe between the arterial valves in relation to the ventricular septum below was associated with outflow obstruction in 14 patients (pulmonary in nine and aortic in five). These features were readily identified by echocardiography. Thus echocardiography is not only a more accurate method than angiocardiography of recognising these defects, but also shows that the arterial pole of the heart is architecturally abnormal.