Cross-sectional echocardiographic studies were used to measure and compare the internal diameter of both aortic root and pulmonary artery in 104 patients with normally related great arteries. Six groups of patients were assessed: normal, with an intracardiac shunt, with tetralogy of Fallot, with pulmonary stenosis, with aortic stenosis, and with atresia of the right atrioventricular valve orifice. In addition, a postmortem study was carried out in normal heart specimens and in specimens with an atrioventricular septal defect. It appeared that the echocardiographically studied ratio between the aortic and pulmonary artery diameter varied considerably. In normal subjects the pulmonary artery diameter tends to be slightly larger than the aortic root diameter: this observation was confirmed by the postmortem data. In other instances the pulmonary artery diameter appeared to be smaller than the aortic root diameter, for example patients with tetralogy of Fallot and those with tricuspid atresia, whereas the reverse was noted in patients with an intracardiac shunt. This two dimensional study has shown that functional adaptations of the calibre of the great arteries can be measured and identified in patients with congenital heart disease.