Although family history and Helicobacter pylori infection are the two main risk factors for gastric cancer, whether eradication of H. pylori benefits those with a family history of gastric cancer was unknown. In a single-centre study in South Korea, 1,676 first-degree relatives of patients with gastric cancer were randomly assigned to receive H. pylori eradication therapy or placebo. After a median follow-up of 9.2 years, gastric cancer developed in 10 individuals in the treatment group versus 23 in the placebo group (P = 0.03), suggesting that H. pylori eradication reduced the risk of gastric cancer in those with a familial risk.