Gastric infection with Helicobacter pylori seems to be a risk factor for gastric cancer. We have conducted a multicentre epidemiological study to investigate this relation further. Our study was designed to look at the relation between the prevalence of H pylori infection and gastric cancer rates in 17 populations from 13 countries, chosen to reflect the global range of gastric cancer incidence. In each centre, about 50 males and 50 females in each of the two age groups 25-34 years and 55-64 years were selected at random from the local population and provided blood samples. Serum samples were assayed for the presence of IgG antibodies to H pylori in a single laboratory. Prevalence rates of H pylori seropositivity were related to local gastric cancer incidence and mortality rates using linear regression. There was a statistically significant relation between the prevalence of seropositivity and cumulative rates (0-74 years) for both gastric cancer incidence and mortality with regression coefficients of 2.68 (p = 0.001) and 1.79 (p = 0.002), respectively. Our findings are consistent with an approximately six-fold increased risk of gastric cancer in populations with 100% H pylori infection compared with populations that have no infection.