We determined the validity of the carbon 13-labeled urea breath test in young children. We found that although the 13C-labeled urea breath test had a specificity greater than 90%, borderline or false positive results occurred more frequently in children younger than 2 years compared with older children. False positive results may be caused by oral-urease-producing organisms because direct intragastric administration of 13C urea reduced the excess delta 13CO2. Care is urged in interpreting one positive 13C-labeled urea breath test in children younger than 2 years.