Cross-sectional echocardiography, combined with injections of contrast into peripheral arm veins, has been used to study 15 patients with atrial septal defects and 10 patients with an intact interatrial septum. Of 11 patients with ostium secundum or sinus venosus atrial septal defects and left-to-right shunts a defect could be visualised in all, and in eight some degree of transfer of contrast from right atrium to left atrium was seen. In three of four patients with a dominant right-to-left shunt a defect was seen and in all there was free transfer of contrast from right atrium to left atrium. Though there may be variable loss of echoes in the septal image in patients with an intact interatrial septum, in general no fixed defect is seen an there is no transfer of contrast from right atrium to left atrium. This is a potentially valuable technique in the assessment of patients in whom an atrial septal defect is suspected.