The frequency of Helicobacter pylori (H.p.) infestation in antral mucosa and the presence of gastritis were investigated in different states of gastric acid secretion. Biopsies were stained by the Warthin-Starry technique and hematoxylin-eosin. Antral H.p. was found in similar frequencies in Zollinger-Ellison syndrome (n = 17; profound acid hypersecretion, associated with duodenal ulcer disease in most cases) and the same number of age-matched controls (35% in each group) whereas H.p. could be detected in 31 out of 33 duodenal ulcer patients (94%). The incidence of H.p. infestation in H2-blocker refractory reflux oesophagitis was low (24%). Treatment of peptic lesions with omeprazole (drug-induced hypochlorhydria) led to a reduction or disappearance of H.p. in 7 out of 10 H.p.-positive patients whereas none of 19 primarily H.p.-negative patients became infected with H.p. during prolonged omeprazole therapy. It is concluded that (1) development of duodenal ulcers (as in gastrinomas) does not necessarily require H.p., and (2) at least in some patients H.p. is reduced in antral mucosa by omeprazole.