It is generally accepted that extensive chronic atrophic gastritis is a precursor to gastric cancer in populations at high risk for this tumor. To improve the effectiveness of gastric cancer screening, we have devised a new screening method that investigates the serum pepsinogen levels and applied a serum pepsinogen test to mass screening for gastric cancer at a certain workplace for the first time. This screening system (named "stomach dry dock") is based on the findings that many gastric cancer develop in the stomach mucosa affected by severe and extensive chronic atrophic gastritis. Using the serum tests (Pepsinogen I/II RIA BEAD Kit: Dainabot Co., LTD. Japan) we have screened 14,862 employees of a certain company in 1991-93 and 25 cases (0.17%) of gastric cancer including 21 cases (84%) of early gastric cancer and 10 cases (0.07%) with gastric adenoma; precancerous lesion. The results are better than that of the traditional barium X-ray screening (incidence of gastric cancer is 0.07%) in the same company. In conclusion the best sensitivity for detecting gastric cancer is achieved by the mass screening with serum pepsinogen tests in combination with X-ray or endoscopy.