It has been shown that serum pepsinogen I levels are correlated with maximal acid outputs and can be used as an indicator for parietal cell mass, In this study, the effect of Helicobacter pylori infection on the relationship between serum pepsinogen I levels and maximal acid outputs was investigated in 27 patients with Helicobacter pylori associated enlarged fold gastritis, Before treatment, serum pepsinogen I levels and maximal acid outputs were not significantly correlated. After eradication of Helicobacter pylori, a significant positive correlation was found between serum pepsinogen I levels and maximal acid outputs with a significant decrease in serum pepsinogen I levels and a significant increase in maximal acid outputs, These results indicate that Helicobacter pylori infection distorts the relationship between serum pepsinogen I levels and maximal acid outputs by elevating the former and lowering the latter, and that serum pepsinogen I level after eradication of Helicobacter pylori may reflect parietal cell mass in patients with Helicobacter pylori associated enlarged fold gastritis.