The distance from the endocardial surface of the atrioventricular valve annulus and adventia of the adjacent coronary artery (CA) has not been delineated and may play an important role in the risk for CA damage during catheter ablation procedures. This distance was measured in 10 normal pediatric heart specimens at 11 locations around the right and left atrioventricular grooves. The endocardium-to-CA distance varied by age and location, with the shortest distances in the region of the os of the coronary sinus and the greatest distances in the right anteroseptal area. Excluding right anteroseptal measurements, in the 5 patients <7 years of age, the endocardium-to-CA distances were <or=4 mm in 76% and <or=5 mm in 96%. Given lesion sizes during ablation procedures, these distances and their potential relation to the risk for CA damage need to be taken into account during catheter ablation procedures in young patients.