Pulmonary vascular structure was studied by analysing serial reconstructions of the arterial pathways and random sections of tissue in the lungs of 16 children who died with different types of congenital heart disease and pulmonary hypertension. Cases of ventricular septal defect showed an appreciable increase in muscularity of both preacinar and intra-acinar (respiratory unit) arteries, and intimal proliferation was infrequent and mild. By contrast, cases of transposition of the great arteries with ventricular septal defect and atrioventricular septal defect showed an increase in preacinar muscularity, a short heavily muscularised arterial segment containing intimal proliferation at the entrance to the acinus, whereas the intra-acinar arteries beyond showed only a moderate increase in muscularity. In these children who were less than 1 year of age an increase in pulmonary vascular resistance was due to strategically placed small areas of intimal proliferation and not to widespread obliterative pulmonary vascular disease. The study demonstrated and explained differences in the appearance of the peripheral pulmonary arteries in different types of congenital heart disease, which help interpret the findings of lung biopsies.