The occurrence of Campylobacter-like organisms (CLO) was studied in the gastric mucosa of 33 patients with gastric ulcer and 33 age- and sex-matched controls with non-ulcer dyspepsia, and the relation of CLO to the severity of gastritis was determined in both groups. CLO was significantly commoner in the ulcer patients (57.5%) than in the non-ulcer cases (33.3%) (p less than 0.05); this was related to a higher frequency of antral atrophic gastritis in the former. It is suggested that CLO may be associated with chronic antral gastritis and hence with gastric ulcer, but the causal relationship remains unclear.