Detection of gastric Campylobacter by the 14C-urea breath test and serology were correlated to biopsy culture in 25 unselected outpatients referred for gastroscopy. All the 17 culture-positive patients had positive 14C-urea breath test, and 16 had positive serology. Of eight culture-negative patients, six patients had negative breath test and seven negative serology. A high degree of reproducibility was found when two subsequent breath tests were performed in 11 healthy volunteers. The breath test values obtained at 10 min showed a strong correlation (r = 0.97, p less than 0.001) to the accumulated values within 30 min. Breath sampling once, 10 min after intake of 2.5 microCi 14C-urea, seems sufficient for the detection of gastric Campylobacter. The 14C-urea breath test correlates well with biopsy culture and provides a sensitive tool for the detection of gastric Campylobacter. Serology also corresponds well with biopsy culture and should provide a useful tool for epidemiologic studies.