Seven adults with ostium primum atrial septal defect (ASD) are presented. All but one patient were symptomatic at the time of diagnosis with the average age being 41 years. At catheterization each had a large, low ASD, left-to-right shunt, and a cleft mitral valve leaflet with minimal regurgitation. Pulmonary hypertension occurred in one. Six patients are asymptomatic an average of 3.5 years after surgical correction. One patient died of other causes at 18 months. Rapid decompensation secondary to arrhythmias or atrioventricular block occurs much more frequently with primum than with secundum ASDs, approaching 80% by age 45. Follow-up in this series suggests that surgical correction may prevent these developments.