Objective: Ventricular septal defects (VSDs), if haemodynamically important, are closed whereas small shunts are left without intervention. The long-term prognosis in congenital VSD is good but patients are still at risk for long-term complications. The aim of this study was to clarify the incidence of infective endocarditis (IE) in adults with VSD. Methods: The Swedish registry for congenital heart disease (SWEDCON) was searched for adults with VSD. 779 patients were identified, 531 with small shunts and 248 who had the VSD previously closed. The National Patient Register was then searched for hospitalisations due to IE in adults during a 10-year period. Results: Sixteen (2%) patients were treated for IE, 6 men and 10 women, with a mean age of 46.3 +/- 12.2 years. The incidence of IE was 1.7-2.7/1000 years in patients without previous intervention, 20-30 times the risk in the general population. Thirteen had small shunts without previous intervention. There was no mortality in these 13 cases. Two patients had undergone repair of their VSD and also aortic valve replacement before the episode of endocarditis and a third patient with repaired VSD had a bicuspid aortic valve, all of these three patients needed reoperation because of their IE and one patient died. No patient with isolated and operated VSD was diagnosed with IE. Conclusions: A small unoperated VSD in adults carries a substantially increased risk of IE but is associated with a low risk of mortality.