OBJECTIVE This study was undertaken to analyze the clinical profile, associated features, and surgical treatments of adults operated on for ostium primum atrial septal defects, particularly factors influencing progression of mitral valve disease. METHODS We retrospectively studied all patients aged 18 years and older operated on at our institution with reference to patient clinical features, investigation findings, surgical records, and outpatient follow-up data. RESULTS Fifty-one patients, 29 female and 22 male, underwent operation at a mean age of 27.3 years (SD 6.9). Of these, 80% were in New York Heart Association functional class I or II, with a most frequent presenting symptom of dyspnea. On echocardiography, 88% had cleft mitral valve, 35% had moderate mitral regurgitation, and 4% had severe mitral regurgitation. According to echocardiography and available cardiac catheterization data, 27% had moderate pulmonary arterial hypertension and 8% had severe. In-hospital mortality was 1.9%. At mean follow-up of 36 months, 94% of patients were in functional class I. Mitral regurgitation was moderate in 21% and severe in 8%, with 1 patient undergoing mitral valve replacement. Factors associated with increased risk of moderate or severe mitral regurgitation on follow-up were preoperative moderate or severe pulmonary arterial hypertension (P = .008) and female sex (P = .009). CONCLUSIONS Surgical correction of ostium primum atrial septal defects in adults can be undertaken successfully with low mortality and excellent symptomatic results. Regular follow-up is required to assess progression of mitral regurgitation, which is more likely in women and those with preoperative pulmonary arterial hypertension.