Purpose: The C-14 urea breath test (C-14 UBT) is the most specific noninvasive method to detect Helicobacter (H) pylori infection. We investigated if the C-14 UBT can reflect the presence and degree of H. pylori detected by gastroduodenoscopic biopsies (GBx). Materials and methods: One hundred fifty patients (M:F=83:67, age 48.6±11.2 yrs) underwent C-14 UBT, rapid urease test (CLO test) and GBx on the same day. For the C-14 UBT, a single breath sample was collected at 10 minutes after ingestion of C-14 urea (137 KBq) capsule and counting was done in a liquid scintillation counter for 1 minute, and the results were classified as positive ( 200 dpm), intermediate (50∼199 dpm) or negative (〈50 dpm). The results of CLO tests were classified as positive or negative according to color change. The results of GBx on giemsa stain were graded 0 (normal) to 4 (diffuse) according to the distribution of H. pylori by the Wyatt method. We compared C-14 UBT results with GBx grade as a gold standard. Results: In the assessment of the presence of H. pylori infection, the C-14 UBT global performance yielded sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV) and accuracy of 92.5%, 88.4%, 97.1%, 88.4% and 91.3%, respectively. However, the CLO test had sensitivity, specificity, PPV, NPV and accuracy of 83.2%, 81.4%, 91.8%, 81.4% and 82.7%, respectively. The quantitative values of the C-14 UBT were 45±27 dpm in grade 0, 707±584 dpm in grade 1, 1558±584 dpm in grade 2, 1851±604 dpm in grade 3, and 2719±892 dpm in grade 4. A significant correlation (r=0.848, p<0.01) was found between C-14 UBT and the grade of distribution of H. pylori infection on GBx with giemsa stain. Conclusion: We conclude that the C-14 UBT is a highly accurate, simple and noninvasive method for the diagnosis of ongoing H. pylori infection and reflects the degree of bacterial distribution. (Korean J Nucl Med 2001;35:61-68)