Diagnosis of Campylobacter pylori gastritis. Campylobacter pylori is a bacterium that inhabits gastric mucosa. It causes chronic active gastritis and is highly associated with duodenal ulcer. Campylobacter pylori has a urease enzyme (not present in man), which allows diagnosis by a [14C]urea breath test. We compared two noninvasive tests, the breath test and serum ELISA, to biopsy and histologic diagnosis. Twenty-two patients who underwent gastroduodenoscopy for evaluation of possible peptic ulcer disease entered the study. The breath test detected the organism in eight of eight patients biopsy-positive for the organism (sensitivity 100%). The breath test was negative in 12 of the 14 patients who were biopsy-negative (specificity 86%). The ELISA was performed in 14 patients. It was positive in 5 of 5 patients biopsy-positive for the organism (sensitivity 100%) and negative in 7 of 9 patients who were biopsy-negative (specificity 78%). We conclude that both the ELISA and the [14C]urea breath test are excellent noninvasive methods to detect Campylobacter pylori. However, only the breath test is suitable for following the response to treatment, as it detects the presence of the organism rather than an immune response to it. 