Eight hundred and four patients with persistence of the ductus arteriosus were seen in Edinburgh between 1940 and 1979. Thirty-seven of them reached the age of 50 years, and in 32 the shunt was exclusively from left to right. Fifteen of the 32 were subsequently treated surgically. None of the 32 was lost to follow-up. Duration of clinical observation averaged 17 years and extended to over 30 years in eight patients. Their features have been correlated with those from reports of 48 comparable patients in an attempt to clarify the management of the persistent ductus in the older patient. Impairment of left ventricular function is shown as the major risk, even when the ductus is small. Bacterial endarteritis is infrequent. Surgical treatment carries greater risk than in childhood and early adult life but usually reduces heart size and restores exercise tolerance. Left ventricular dysfunction, however, occasionally vitiates the benefits; symptoms are then incompletely relieved and death from heart failure may occur months or years after operation. Experience in older patients thus emphasises the value of elective operation in childhood, however well the child, however trivial the shunt. It is concluded that in older patients, the presence or the development of symptoms or cardiac enlargement are almost always indications for surgical treatment. As age increases, especially by the eighth decade, medical treatment may be preferable. Continued follow-up of symptomless patients without cardiomegaly is important because increase in heart size usually precedes further deterioration which can then be prevented by timely surgical treatment.