Nerves containing peptides that supply the human intrapulmonary vasculature were studied in 21 controls aged one month to 24 years and in 13 patients with pulmonary hypertension aged 11 days to eight years. An indirect immunofluorescence technique was used to study the distribution and relative density of nerve fibres containing the general neuronal marker, protein gene product 9.5; tyrosine hydroxylase; synaptophysin; neuropeptide tyrosine; vasoactive intestinal polypeptide; substance P, somatostatin; and calcitonin gene related peptide. At all ages in normal and hypertensive lungs neuropeptide tyrosine was the predominant neuropeptide associated with the pulmonary vascular nerves. In normal lungs the relative density of nerve fibres increased during childhood only in the arteries of the respiratory unit. Pulmonary hypertension was associated with the premature innervation of these arteries during the first year of life. Innervation of small, abnormally thick-walled pre-capillary vessels by predominantly vasoconstrictor nerves may help to explain the susceptibility of infants to pulmonary hypertensive crises.