Balloon dilatation in infants with severe pulmonary valve stenosis may not be a straightforward procedure once the arterial duct has closed. Balloon dilatation was attempted in three neonates and infants. In an 11 week old infant hypotension and bradycardia developed shortly after a 5 French end hole catheter was passed through the severely stenosed pulmonary valve. An emergency Waterston shunt was subsequently performed, but he died three days later. After this experience the technique was modified so that progressively larger balloons were used for dilatation in two infants, aged one and three weeks, with severe pulmonary valve stenosis in whom the arterial duct had closed. It was successful in both.