Chronic haemolysis is a rare but important postsurgical complication in children with congenital heart defects. A 13-year-old with a previously uncorrected complex heart defect underwent corrective surgery. He developed haemolysis postoperatively. A mechanical cause due to turbulent flow through a narrowed intraventricular Dacron patch tunnel was suspected. Balloon dilatation and stent placement successfully resolved the stenosis and haemolysis.