The atrioventricular node is situated in the lower atrial septum, at the apex of the Koch's triangle. The dimensions of the Koch's triangle are studied in adult humans, while no data exist about them in pediatric age. The knowledge of the dimensions of Koch's triangle in childhood is very important for safe and correct application of radiofrequency energy during transcatheter ablation. The dimensions of Koch's triangle were determined in 69 human pediatric hearts. The median age of the children was 3 months, with a range from 1 day to 14 years, 30 were female and 39 were male. Relations between body weight (extracardiac parameter) and tricuspid valve diameter (intracardiac parameter) were determined in all hearts to show morphometric modifications with growth. The distribution of body weight was not Gaussian and no correlation could be obtained between Koch's triangle dimensions and body weight. However, it was possible to identify that the mean ratio between the cathetus of the Koch's triangle corresponding to the annulus of the tricuspid valve and the tricuspid valve diameter was 0.45 +/- 0.16, with a highly significant correlation coefficient (r = 0.653, P < 0.001). Therefore, by knowing: (1) the diameter of the tricuspid valve, and (2) the constant ratio between the cathetus of the Koch's triangle and the tricuspid valve diameter, it is possible to calculate the length of the segment of the tricuspid annulus along which the transcatheter application of radiofrequency current can be applied to ablate the slow-pathway, thus reducing the risks of damage of the atrioventricular node.