To determine the neonatal body measurement that best predicts insertional umbilical catheter lengths, we studied two commonly used parameters--shoulder-to-umbilicus length and total body length--and two new parameters--suprasternal notch-pubic symphysis length and birth weight (BW)--on 43 umbilical artery (UA) and ten umbilical vein (UV) catheterizations. The new parameters showed better correlations than the currently used ones. The BW variable, with the highest coefficient of correlation among all four parameters, was applied prospectively in 25 UA and 16 UV catheter insertions. All catheter tips thus placed were found to be in acceptable positions. Although any one of the four parameters can be used clinically, we prefer the BW parameter for its reliability, reproducibility, and ability to be estimated in emergency settings. The modified regression (estimating) equations utilizing BW are as follows: UA catheter length = 3 X BW + 9; UV catheter length = 1/2 X UA line calculation + 1, in which lengths are measured in centimeters and BW is measured in kilograms.