The systemic circulation of newborn infants with congenital left-heart obstruction is supplied from the right ventricle via a patent arterial duct between the pulmonary artery and descending aorta. The duct closes during the first few days of life, but infusion of prostaglandin E2 can prevent closure in some cases. We report four newborn infants (aged 3-8 days) with intractable heart failure due to severe obstruction of the left heart in the presence of a closing arterial duct. Infusion of prostaglandin E2 did not improve their clinical condition. Cardiac catheterisation and balloon dilatation of their arterial ducts resulted in a dramatic improvement in the babies' clinical condition; during subsequent surgical repair of the infants' hearts, the arterial ducts were found to be widely patent. Balloon dilatation gives immediate and sustained wide patency of the arterial duct in infants who do not respond adequately to prostaglandin E2.