Altogether 301 patients with duodenal, gastric and prepyloric ulcers and non-ulcer controls were examined to study whether the Campylobacter pylori is related to ulcer itself or to coexisting chronic gastritis. Histological sections from antral and body mucosa were stained according to Warthin-Starry, Giemsa and with hematoxylin-eosin. The Campylobacter pylori was strongly associated with chronic superficial gastritis in both ulcer and non-ulcer patients and in both antral and body mucosa. No differences were found in the frequency of Campylobacter-positive cases between ulcer patients and non-ulcer controls when the comparison was made within this category of chronic gastritis (p greater than 0.05). The gastric ulcer patients have more often than non-ulcer controls the Campylobacter pylori in atrophic body gastritis (p less than 0.001). The bacteria were only occasionally seen in normal mucosa.