Out of 709 consecutive patients with isolated secundum atrial septal defect, the pulmonary artery systolic pressure was greater than 50 mm Hg in 118 patients (17%). Pulmonary hypertension was present in 13% of patients under 10 years and in 14% aged 11 to 20 years. The Eisenmenger reaction was present in 9% of the 709 patients. The frequency of the Eisenmenger reaction was high in young patients and was not significantly different in patients in the first and second decades as compared to older patients. None of our patients with pulmonary hypertension resided at high altitude. The high frequency of pulmonary hypertension in our young patients cannot be satisfactorily explained. Autopsy studies suggest that in some, pulmonary hypertension is due to the persistence of the fetal pulmonary vascular pattern.