Background Primum atrial septal defect (ASD) is a defect that is usually associated with few symptoms and can be electively repaired with good results.A review of our experience with primum ASD identified a subset of patients characterized by presentation with congestive heart failure (CHF) in the first months of life and a requirement for early operation. Echocardiograms, catheterizations, operative reports, and clinical data were reviewed and compared with that on all other patients with primum ASD. Patients with transitional or complete atrioventricular (AV) canal defects were excluded.Methods and Results From January 1984 to December 1992, significant CHF was present in the first year of life in 11 patients (10.5%) with primum ASD who were managed surgically among 105 total patients undergoing repair of primum ASD. This early CHF group had a higher incidence of hypoplastic left-sided cardiac structures (9 of 11 patients) compared with other primum ASD patients (5 of 94, P<.001), including patients with coarctation (CoA) (n=9), abnormal mitral valve (n=7), left ventricular hypoplasia (n=5), and subaortic stenosis (subAS) (n=7). Other differentiators from patients without early CHF included incidence of Downs syndrome (0% versus 19%), elevated pulmonary artery pressures (72% versus 33% systemic, P<.001), earlier mean age at operation (8 months versus 5.4 years), greater incidence of reoperation after initial ASD closure (5 of 8 patients, 12 operations (5 subAS, 3 mitral valve, 3 CoA, 1 pacemaker) versus 4 of 93 patients, P<.001), and higher mortality (36% versus 1%, P<.001).Conclusions Left-sided obstructive lesions must be sought in children with primum ASD presenting with CHF in the first year of life. The presence of these lesions alters prognosis and surgical management and mandates close follow-up, with particular attention to late appearance or progression of subaortic stenosis or deterioration of mitral valve function. (Circulation. 1994;90(part 2):II-30-II-35.)