We conducted a prospective study to evaluate the relationship between Helicobacter pylori infection and chronic gastritis in a Japanese population. H. pylori was found in 67% of patients positive for mononuclear cell (MN cell) infiltration and in 75.8% of patients positive for polymorphonuclear cell (PMN cell) infiltration, whereas H. pylori was found in 14.2% of patients without MN cell infiltration and in 33.2% of patients without PMN cell infiltration. The frequency of MN and PMN cell infiltration in H. pylori-positive patients was significantly higher than that in H. pylori-negative patients. The frequency of H. pylori infection did not differ in those with atrophic gastritis from those without, whereas the frequency of intestinal metaplasia became significantly higher in those with moderate and severe atrophy. There was no significant difference between serum pepsinogen I (PG I) levels in H. pylori-positive and -negative patients. However serum PG II levels were significantly increased in H. pylori-positive patients aged 40-69 years. The serum PG I-II ratio was significantly lower in H. pylori-positive patients aged 40-59 years. These results suggest that H. pylori infection in the gastric mucosa plays an important role in the pathogenesis of chronic and atrophic gastritis and in the development of intestinal metaplasia.