Helicobacter pylori (H. pylori), ex-Campylobacter pylori, is now considered to be the causative agent of active chronic gastritis in humans and may be diagnosed by histology, gram-stain, culture, urease test or noninvasively by 14C-urea breath test. The aim of this study was to determine the sensitivity, specificity, and reproducibility of the breath test as compared with the microbiological (culture) "gold standard". Forty-one subjects were studied. Gastroscopy was performed in all participants and 2 antral mucosa biopsy specimens were taken for urease test and microbiological (gram-stain and culture) studies. The breath test was performed within one week after the gastroscopy. After fasting overnight, a control breath sample was obtained and a standard meal was offered to delay gastric emptying. Then, the participants drank 5 microCi of 14C-urea in 20 ml of water and breath samples were collected at 10, 15, 30 and 60 min in a hyamine-ethanol solution with a p-H indicator. A repeat test was performed in 9 subjects to assess test reproducibility. Twenty-nine subjects (71 percent) were Hp positive and 12 (29 percent) H. pylori negative. A highly significant difference (p less than 0.0001) was observed in 14CO2 exhalation in colonized and non-colonized subjects in samples collected at all times studied. As compared with culture we found that the breath test was 97 percent sensitive and 100 percent specific. We conclude that 14C-urea breath test is a simple, inexpensive, sensitive, and reliable method to detect H. pylori colonization. It is a useful test to epidemiologic and therapeutic trials.