Knowledge of the morphometric quantities of the coronary arteries in neonates is an increasingly vital component in the management of congenital and acquired heart disease. Because of the considerable heterogeneity of coronary vasculature, what is considered atypical and aberrant or insignificant anatomy is often unclear. The purpose of our present study is to define the normal anatomy of neonates. This was done by focusing on segment analysis of the coronary arteries, which was used to obtain accurate definitions of the length and diameter of the coronary network. The lengths, widths and numbers of collateral branches of the coronary arteries of neonates were measured. The coronary vessels of 50 neonate hearts were visualised post mortem by injection of the coronary arteries with opaque X-ray dye for the imaging study. Black ink cast and silver impregnation specimens were also studied. The longest segment of the circumflex branches of the left coronary arteries was the first, the lengths measuring 7188.5 +/- 839.6 microm and the diameters 850 +/- 90.8 mum. The lengths of segments II, III and IV were 5780 +/- 1182.7 microm, 5397.5 +/- 2070.2 microm and 6932.8 +/- 2236.5 microm and the diameters were 680 +/- 90.8 microm, 510 +/- 90.8 microm and 408 +/- 77.58 microm respectively. The longest segment of the anterior interventricular branches of the left coronary arteries was the first, with lengths of 10151.4 +/- 1298.6 microm and diameters of 1141.9 +/- 82.1 microm. The lengths of segments II, III and IV were 8208.5 +/- 1222.3 microm, 3278.5 +/- 602.4 microm and 5370 +/- 1657.6 mum and the diameters were 971 +/- 82.1 microm, 801.42 +/- 82.1 microm and 631.4 +/- 82.1 microm respectively. The lateral branches were increased in number in the fourth segment. Its diameters ranged from 157.8 +/- 31.7 microm to 655.7 +/- 99.7 microm. The main branch of the right coronary artery was short at the base of the heart. In the newborn the lateral branches of the right coronary artery were short, scattered and curved. Analysis of the data suggests a new anatomical system for classifying the vasculature of the coronary arteries in neonates.