The 13/14C-Urea breath test (UBT) is based on the simple principle that a solution of isotopically labelled urea will be rapidly hydrolysed by the abundantly expressed urease of H pylori. The released 13/14CO2 is absorbed across the mucus layer to the gastric mucosa and hence, via the systemic circulation, excreted in the expired breath. Distribution of urea throughout the stomach prevents sampling error and allows semiquantitative assessments of the extent of H pylori infection. Originally the 13C-UBT was complex, cumbersome and costly but, by simplifying the protocol and reducing the number of samples to be analysed, is now a much easier, quicker and cheaper test for detecting H pylori. Although mass spectrometry is needed for analysis of exhaled 13CO2, the use of the stable isotope, which is completely safe, provides advantages over the 14C-UBT using radioactive 14C-urea, such that it can be used in women and children and a user's licence is not required. The widespread availability of scintigraphy for 14CO2 analysis may make the 14C-UBT seem an attractive alternative to the 13C-UBT. However, there are no standard protocols for the 14C-UBT and although the methods are similar, several different cut off values are used which makes formal validation studies still necessary. Both tests are easy to perform with minimum opportunity for observer variation or methodological error; they are very sensitive and specific tests and provide a clinical "gold standard" against which the accuracy of other tests can be validated. The 13/14C-UBT detects only current infection and can be used to screen for H pylori infection and as the sole method for assessing eradication. In addition, because the 13C-UBT can be performed repeatedly in the same subject, it can be used to monitor the effects of novel anti-H pylori therapies and for epidemiological studies in children.