A nested case-control study was carried out to investigate the association between Helicobacter pylori infection and gastric cancer risk in Taiwan. A total of 29 patients newly affected with gastric cancer and 220 healthy controls matched with cases on age, sex and residence were selected from a cohort of 9,775 men recruited from 1984 through 1986. Frozen serum samples collected at recruitment examination were tested for IgG antibodies against Helicobacter pylori by an enzyme-linked immunosorbent assay. The average interval between serum collection and cancer diagnosis was 3.1 years. Gastric cancer cases had a higher seropositive prevalence (69%) than matched controls (59%) giving an odds ratio of 1.6 (95%) confidence interval = 0.7-2.6). Compared with previous nested case-control studies, Helicobacter pylori infection in early childhood may be a risk factor for gastric cancer. However, a long induction period seems required for gastro-carcinogenesis associated with Helicobacter pylori infection.