Preoperative distinction between common atrioventricular orifice and ostium primum atrial septal defect may be difficult. To improve diagnostic accuracy, the right and left ventricle angiocardiograms were reviewed 'blind' in 92 patients with atrioventricular defects. The true diagnosis was known from necropsy or surgery in 60. Angiocardiograms had been obtained in various projections with or without craniocaudal tilt. Those features thought to distinguish between common orifice and ostium primum were coded, together with the ventricular systolic pressures. Computerised disciminant function analysis identified the following distinguishing features: (1) right ventricular systolic pressure; (2) immediate right ventricular outflow tract opacification from the left ventricle; (3) identification of the anterior attachment of the mitral component; (4) recognition of a single straddling atrioventricular orifice; (5) passage of contrast medium above or below the anterior or posterior bridging leaflets. Feature (3) indicates that in contrast to classic teaching the direct septal attachment of the mitral component does not contribute to the 'gooseneck' in complete atrioventricular defects. The significance of (4) and (5) is that they may be identified from right as well as left ventriculography, and are more likely to be identified in oblique than standard projections. Computerisation produced a correct diagnosis in 92 per cent of known cases, and determined precise probabilities of diagnosis in the remainder.