Interventional balloon catheterisation is now the recommended procedure for several congenital heart defects. The overall complication rate in the first 160 children (median age 3 years) to undergo balloon intervention (excluding balloon atrial septostomy) at the Brompton Hospital was 24% (14% excluding haemorrhage and including three deaths (1.9%]. Most complications were related to vascular problems at the site of cardiac catheterisation. Complications were more common in the younger patient, especially neonates, and also in procedures that were ultimately unsuccessful. Most complications were transient and usually had no long term effects. Improvements in equipment design and technical expertise may reduce morbidity from these procedures.