Two-dimensional echocardiographic findings in a group of 24 patients with atrial septal defects were correlated with findings obtained by cardiac catheterization and M-mode echocardiography. The prevalence of mitral prolapse was 95% by two-dimensional echocardiography and 59% by angiography in patients with secundum and sinus venosus atrial septal defects. The majority of the group with echocardiographic prolapse manifested a distinctive pattern of prolapse, with predominant involvement of the anterior mitral leaflet. Thirty-eight percent of the patients in this series manifested paradoxical septal motion by M-mode and/or two-dimensional echocardiography. In the patients with abnormal septal motion, the net systolic anterior movement of the septum was caused by an exaggerated systolic anterior movement of the main body of the left ventricle. The atrial septal defect could be visualized with confidence by two-dimensional echocardiography only in the two patients with ostium primum atrial septal defects. Cleft anterior mitral leaflets were also clearly demonstrated in these two patients.