This report describes the long-term (up to 38 years) outcome, in terms of mortality and cardiovascular morbidity, in a non-randomized clinical observation study of a surgically and a conservatively managed group of adult patients with shunt lesions at atrial level. Haemodynamic status was assessed at baseline and at repeat examination. Twelve patients underwent surgical repair of the lesion between these two heart catheterizations, resulting in a marked reduction in heart size and right ventricular systolic pressure. These changes were associated with improvement in functional class and a trend towards less risk of cerebrovascular incidents, but not atrial fibrillation, during follow-up. Among 12 patients not operated on between catheterizations, symptomatic deterioration was common, often necessitating later surgical repair. The results support the assumption that early surgery should be recommended for adults with a haemodynamically significant lesion, to reduce the risk of mortality and prevent symptomatic deterioration.