Campylobacter pyloridis was isolated from 77% of 220 (35%) unselected adults undergoing gastroscopy. Isolation was significantly associated with histological gastritis (p less than 0.0001), duodenal ulcer (p less than 0.0001), and to a much lesser extent, with gastric ulcer (p less than 0.05). The relation between the isolation of C pyloridis and peptic ulcer seemed to be independent of coexisting gastritis. In those with no endoscopic or histological evidence of disease there was no relation between isolation and increasing age. Antibody responses to a whole cell sonicate of a strain of C pyloridis were measured by means of an enzyme linked immunosorbent assay (ELISA). Increased IgA (p less than 0.0001) and IgG (p less than 0.0001) antibody titres were found in patients with C pyloridis. Peptic ulceration or gastritis were present in 78% and 100% of patients with a high concentration of IgG and IgA, respectively, but in only 9% and 18% of those with low titres. These results provide further evidence for a possible pathogenic role of these organisms in gastric disease and suggest that immunological markers of their presence might be useful non-invasive indicators of disease.