Campylobacter pyloridis was ingested by a volunteer who had a histologically normal gastric mucosa and fasting gastric pH recordings of less than 2. Three days later he developed moderate to severe attacks of epigastric pain. On the 5th day after ingestion C. pyloridis was cultured from antral biopsies which showed histological acute gastritis. However, fundal histology and fasting gastric pH were normal. On the 8th day fasting gastric pH rose to 7.6. On day 11 C. pyloridis was cultured from both antral and fundal biopsies which showed histological gastritis. Doxycycline was taken for 28 days but infection and gastritis persisted. Bismuth subsalicylate was taken for 28 days and final biopsies, taken 1 wk after stopping therapy, were culture negative. Histology showed a minimal residual chronic gastritis. C. pyloridis can cause an acute upper gastrointestinal illness associated with histological gastritis and an increase in fasting gastric pH.