The relations between tobacco, alcohol and risk of gastric cancer need to be established, and any gain from preventive measures should be estimated. We conducted a population-based, prospective cohort study in Nord-Trondelag county in Norway. During 1984-1986, adult residents were invited to a health survey and they answered questionnaires that assessed exposure to tobacco and alcohol, together with potential confounding factors. The exposure assessment regarding alcohol was limited to a 14-day period. New gastric cancers that occurred during follow-up (1984-2002) were identified by linkage to the Norwegian Cancer Registry. Cox proportion hazards regression models were used to calculate hazard ratios (HRs) with 95% confidence intervals (CI), adjusted for sex, education and body mass index. Follow-up of 1,117,648 person-years at risk among 69,962 cohort members revealed 251 gastric cancers, including 224 noncardia cancers. The risk was almost twice as high in daily smokers (HR = 1.88 [CI 95% = 1.33-2.67]) as in never smokers. Independent dose-response relations were found with earlier age at initiation (p = 0.02), frequency (p = 0.00) and duration of smoking (p = 0.00). Attributable risk (AR) of gastric cancer among current smokers was 8.7/100,000 person-years and the corresponding population AR was 18.4%. No statistically significant associations between various degrees of exposure to alcohol and risk of gastric cancer was revealed, but combined high use of cigarettes (>20/day) and alcohol (>5 occasions/14 days) increased the risk of noncardia gastric cancer nearly 5-fold (HR = 4.90 [95% CI = 1.90-12.62]), compared to nonusers. It is concluded that smoking is a dose-dependent risk factor for gastric cancer. Combined high exposure to smoking and alcohol further increases the risk. Successful preventive measures could considerably reduce the incidence of gastric cancer.