The haemodynamic effects of infusion of epoprostenol (prostacyclin) and bolus injection of tolazoline were compared in a crossover study in 11 children with pulmonary hypertension caused by pulmonary vascular disease. The children were studied during cardiac catheterisation, while they were anaesthetised, paralysed, and ventilated with 100% oxygen. The order of drug administration was not randomised because tolazoline has a half life of hours whereas epoprostenol has a half life of a few minutes. Both drugs caused pulmonary and systemic vasodilatation, and there were no significant differences between the two. The 95% confidence intervals suggest that tolazoline did not have a clinically important haemodynamic advantage over epoprostenol. Previous reports suggest that serious side effects are common when tolazoline is used in repeated doses; epoprostenol has only a few minor side effects that are rapidly reversible when the infusion is stopped. Epoprostenol is more expensive than tolazoline but this study suggests that epoprostenol is a more suitable pulmonary vasodilator if more than a single dose is required.