Chronic atrophic gastritis (CAG) is closely correlated with gastric cancer and is predominant in Japan. Epidemiologically, food habits are the primary factor in both CAG and gastric cancer. Two potential serum markers for CAG have recently been investigated, i.e., the concentration of serum pepsinogen (PG) and the presence of serum antibodies against Helicobacter pylori. Serum PG I and II and the PG I: PG II ratio have been reported to be useful as indicators of recurrent peptic ulcer and for screening of patients at risk from gastric cancer. In this study, we examined PG I and II in serum from 483 patients by RIA (DAINABOT), and endoscopic examination performed in the same patients before serological assay revealed CAG in 68, peptic ulcer in 91, and gastric cancer in 48. Analysis of the mean values according to patients age showed that CAG patients in their forties to eighties had low (<40 ng/ml) levels of PG I, peptic ulcer patients in their teens to eighties had high (>70 ng/ml) levels, except for those in their seventies, and gastric cancer patients in their twenties to sixties had low (<3.0) PG I:PG II ratios, except for those in their sixties. Thus serum PG assay has potential utility for detection of CAG, peptic ulcer, and gastric cancer.