The records of 264 pediatric patients with uncomplicated ostium secundum atrial septal defect (ASD) were reviewed. Eighty-seven patients were younger than age 4 years at the time of cardiac catheterization. Subnormal weight gain, frequent pneumonia, cyanosis or tachypnea were present in 26 patients (30%). Of the 36 infants at catheterization, 17 (48%) had the previously described symptoms, including 12 (33%) who had congestive heart failure. Eight of the 36 infants were found to have closed their defect at a subsequent catheterization. Six of 18 patients who underwent cardiac catheterization between 1 and 2 years of age also had spontaneously closed their ASD at subsequent study. Statistical analysis of hemodynamic data revealed no difference (except a smaller shunt size) between ASDs that closed and those that did not in patients who were <4 years at initial catheterization. Analysis of hemodynamic data revealed no statistical differences between groups of patients with an ASD who were younger than and those older than 4 years at time of diagnostic study. Patients with ASDs that closed were significantly different from patients with atrial level shunting thought to be secondary to a valve-incompetent foramen ovale with respect to age at initial study (11 versus 2 months, p <0.001), mean left atrial pressure (7.7 versus 12.3 mm Hg, p <0.02) and difference between mean right and left atrial pressures (1.0 versus 4.2 mm Hg, p <0.01). Because of this high incidence of spontaneous closure of ASD, even in symptomatic patients, intensive medical management rather than surgical correction is recommended for patients with ASD who are younger than age 2 years. Elective surgical repair of ASD diagnosed after age 4 years is recommended because spontaneous closure after that age is probably unlikely.