There is a relation between excess body mass index and overall cancer incidence and mortality, but the relation to noncardia gastric cancer is inconsistent. A high physical activity level decreases the risk of several cancers, but few studies have focused on gastric cancer. We conducted a population-based, prospective cohort study in Nord-Trondelag county in Norway. During 1984 to 1986, all adult residents were invited to participate in a health survey, where body height and weight were measured, and frequency, duration, and intensity of recreational physical activity, together with potential confounding factors, were assessed by questionnaires. New gastric cancers occurring during follow-up in 1984 to 2002 were identified by linkage to the Cancer Registry of Norway. Cox proportional hazards regression models were used to calculate hazard ratios with 95% confidence intervals, adjusted for age, sex, occupation, salt intake, smoking, and alcohol drinking. Follow-up of 73,133 cohort members (88% of all inhabitants) revealed 313 gastric cancers, including 264 noncardia cancers. No statistically significant associations were revealed between different levels of body mass index and risk of noncardia gastric cancer. A statistically significant 50% risk reduction among persons reporting at least a moderate level of recreational physical activity, based on a summary score of physical activity (hazard ratio, 0.5; 95% confidence interval, 0.3-0.9), and a dose-response relation was indicated (P for trend = 0.01). It is concluded that recreational physical activity might have a protective effect against gastric cancer. The sedentary lifestyle gaining ground in western societies might counteract the ongoing decrease in incidence of gastric cancer.