Pulmonary vascular structure was analyzed in the lungs of 10 patients with a secundum atrial septal defect (ASD) in whom pulmonary hypertension had developed. Four patients were aged 6 months or less, 5 were aged 2 to 9 years, and 1 was 21 years old. Pulmonary vascular structure was analyzed using lung biopsy tissue in 5 and autopsy material in the other 5. All the infants presented with heart failure and all had a marked increase in pulmonary arterial smooth muscle; only 1 infant survived surgery. Of the 5 older children, 1 presented with cyanosis, but in the rest the ASD was incidental to the presentation. Three patients had severe pulmonary vascular disease, similar to that seen in adults with a hypertensive ASD. Only 2 older children underwent successful surgery. In 1 child and in the 1 adult, the severity of the pulmonary vascular disease precluded surgery. The ASD was closed in 8 patients, but only 3 survived. Pulmonary hypertension develops rarely in secundum ASD in childhood.