The presence of Campylobacter pylori was investigated in gastric antral biopsy specimens. In 50 consecutive patients undergoing upper gastrointestinal tract endoscopy microbiological cultures, histological examination and rapid urease test were parallel performed, and a 92 per cent sensitivity and 100 per cent specificity of rapid and cheap urease test were determined. Afterwards--in a prospective study--311 patients were examined for C. p. by the rapid urease test only. C. p. was detected in 92 per cent of duodenal ulcer patients, in 52 per cent of patients with gastric ulcer, in 67 per cent of non-ulcer dyspepsia, in 62 per cent of mixed diabetic patient material, and in 21 per cent only of asymptomatic volunteers. It has been found by the authors, that the rate of C. p. infection increased parallel with the continuance of diabetes and did not follow the increasing with age as in the general population. This is the first observation in the world literature concerning the correlation between C. p. and diabetes mellitus. Very close, significant correlation has been found between C. p. infection and chronic active gastritis. C. p. may play an important role in the recurrences of duodenal ulcer and in the pathogenesis of non-ulcer and diabetic dyspepsia. Further studies are planned to the correct evaluation of pathogeneity of Campylobacter pylori.