A breath-test has been developed for the detection of gastric infection with Campylobacter pylori. Urea that is labelled with carbon 14 is administered to a fasting patient and the patient's breath is sampled for radioactivity over the following 30 minutes. If C. pylori is present in the patient's stomach, urease activity causes hydrolysis of the urea and the 14C is absorbed as carbon dioxide. This carbon dioxide enters the patient's bicarbonate pool and eventually is excreted in the breath. The results are expressed as a percentage of the administered dose/mmol carbon dioxide x kg body weight. Sixty-three patients who were undergoing endoscopy were studied. The radioactivity in exhaled breath which was sampled within five minutes of 14C-urea administration was attributed to the presence of urease enzyme in mouth organisms and was discounted. The time-radioactivity curves for breath samples from five to 30 minutes after the administration of 14C-urea gave an excellent separation between subjects with negative results of the examination of gastric-biopsy samples and patients with microbiological and histological evidence of infection with C. pylori. The area under the time-radioactivity curve at between five and 30 minutes after the administration of 14C-urea in 24 patients with negative microbiological results was 6.9 +/- 4.4 area units; in 35 of 39 patients with positive microbiological results, this area was greater than 40 area units. Measured against the results of the microbiological examination of gastric-biopsy samples, the sensitivity of breath-testing was 90% and the specificity was 100%. Measured against the results of histological examination for the presence of C. pylori infection, breath-testing had a sensitivity of 94% and a specificity of 93%. A positive breath-test result also correlated well (P = 0.0001) with the serological antibody test-result. The role of non-invasive tests--enzyme-linked immunosorbent assays and 14C-urea breath-testing--in the management of gastritis and peptic ulcer disease is discussed. We consider that the 14C-urea breath-test has an important role in the noninvasive confirmation of gastric infection with C. pylori and in the follow-up of patients after treatment.