A survey of the prevalence of Campylobacter pylori in an unselected series of 117 patients referred for gastroscopy showed that 49.6% of the patients were infected. In 87% of infected patients both the antrum and the body of the stomach were involved. Duodenal infection occurred in only 9% of 102 studied patients. C. pylori could usually be demonstrated in mucosa showing signs of active chronic inflammation, whereas it was rarely found in histologically normal mucosa. The acridine orange staining method for demonstration of bacilli correlated well with bacteriology; identical results were obtained in 92.8% of the samples.