Helicobacter pylori (Hp) is a major risk factor for gastric carcinogenesis. In Japan, the incidence of Hp infection in young adults has declined markedly. The purpose of this study was to clarify this trend in the incidence of Hp-associated gastric carcinoma (GCa) in young (under 30 years of age) Japanese patients. We enrolled 53 such patients who underwent surgical resection of GCa in one of 18 hospitals in the Hiroshima prefecture between 1976 and 1999. The patients were classified into groups based on three 8-year periods (1976-83, 1984-91, and 1992-1999) in which their cases occurred. We compared the numbers of patients and estimated the histology of carcinomas, grades of gastritis and Hp infection histologically. Of the 53 patients, 49 (92%) showed Hp infection. The frequency of GCa in young adults has decreased gradually (21, 18, and 14 patients in 1976-83, 1984-91 and 1992-99, respectively). The numbers of Hp-positive carcinomas decreased radically (21, 17 and 11 patients, respectively). This trend was associated with improvement in the degree of gastritis in non-neoplastic mucosa. Of the four Hp-negative patients, three had signet ring cell carcinoma. Moreover, the numbers of patients with non-signet ring cell carcinoma also decreased (18, 12 and 7 patients in each period, respectively). These results suggest that Hp-associated carcinoma has declined gradually in young Japanese.