Pulmonary vascular changes were studied in histological sections from 15 children and 25 adults with primary plexogenic arteriopathy. The severity of medial hypertrophy and degree of vasoconstriction were measured in histological sections and there was a close correlation between these two variables in both children and adults. More advanced arterial changes, expressed as an index of pulmonary vascular disease, were more common in adults, and their severity correlated positively with the degree of medial hypertrophy. No such correlation was found in children. There were similar numbers of plexiform lesions per square centimetre in children and adults, so that the differences in the indices of pulmonary vascular disease were mainly due to the intimal changes. Concentric laminar intimal fibrosis was more severe in adults. It is suggested that intensive spastic vasoconstriction results in the development of fibrinoid necrosis and subsequently of plexiform lesions and that this may happen irrespective of the presence of severe intimal fibrosis. This suggests that children with primary plexogenic arteriopathy in whom plexiform lesions have not yet developed are more likely to respond to vasodilator treatment than are adults in whom irreversible changes associated with intimal fibrosis have developed.