Fatal haemopericardium occurred in a 14-year-old boy after rupture of a pulmonary artery aneurysm. Persistent ductus arteriosus with severe pulmonary hypertension was confirmed by cardiac catheterisation when he was 4 weeks old. Attempted closure of the ductus at 4 years had not been possible because of apparent high pulmonary resistance. Exercise tolerance had been good enough to permit competitive horse riding up to the day of death. Light and electron microsocpy showed widespread cystic medionecrosis of the elastic pulmonary arteries.