Helicobacter Pylori (HP) were found in 878 (73%) of 1205 patients undergoing upper G-I endoscopy with multiple biopsies for gastroduodenal diseases. HP were present in similar percentages among patients with active (89%) or healed (81%) peptic ulcer as well as in non ulcerous dyspeptics affected with gastritis (85%). 96% of active chronic gastritis were infected by HP as compared with 55% of quiescent gastritis. Antral gastritis was more frequently active in patients with ulcer diseases (76%) than in dyspeptic and asyntomatic patients (50%). Healed gastric and duodenal ulcers showed decreased incidence of active antral gastritis (69) as compared with active ulcers. Conversely body gastritis was more frequently active in healed (37%) than in overt (18%) duodenal ulcers. 95 histologically normal stomachs as well as 9 cases exhibiting type A gastritis were devoid of HP. High rates of infection were found in 610 cases of chronic gastritis without atrophy as well as in 151 atrophic antral (type B) gastritis. Cytoplasmic vacuolization and swelling of foveolar-superficial cells with adhering bacteria, micropapillae and microerosions were commonly found in HP-infected mucosa. In 16 of 19 children with type B chronic gastritis antibacterial therapy eradicated HP. This was followed by resolution or striking improvement of gastritis and disappearance of epithelial lesions.