Localizing the bleeding site in pediatric patients with gastrointestinal hemorrhage may require invasive and costly diagnostic procedures. A simple index to discriminate upper and lower bleeding sources would be invaluable. We evaluated the reliability of the calculated blood urea nitrogen/creatinine (BUN/Cr) ratio in segregating upper from lower gastrointestinal bleeding sites in 40 children. For upper gastrointestinal hemorrhage, the calculated BUN/Cr ratios (mg/mg) ranged from 10 to 140, with a mean value of 34. For lower gastrointestinal bleeders, the BUN/Cr ratios ranged from 3.3 to 30, with a mean value of 16. All BUN/Cr ratios greater than 30 corresponded to patients with documented upper gastrointestinal bleeding sources. Calculation of the BUN/Cr ratio in the initial evaluation of gastrointestinal bleeding may prove useful in guiding the sequence of diagnostic procedures and examinations.