['The importance of tobacco smoking and Helicobacter pylori infection as risk factors in the development of gastric carcinoma was investigated through multivariate conditional logistic regression analysis in a nested case-control study.', 'Blood samples and a questionnaire on smoking habits were collected from a cohort of 32,906 city residents during a health screening programme from 1974 to 1992. Fifty-six cases of gastric cancer and 224 matched controls were selected. The mean interval between screening and cancer diagnosis was 5.7 years. H. pylori infection was determined by IgG-serology. Occupation categorized into blue-collar workers, white-collar workers, self-employed and unknown occupation was included in the statistical analysis as an indicator of socio-economic status.', 'The proportion of current smokers was 61% among gastric cancer cases, versus 41% among controls. H. pylori seropositivity was present in 82% of the cases and 49% of the controls. In a multivariate model current smokers had an odds ratio (OR) of 2.2 (95% confidence interval (CI): 1.2-4.2). With different levels of tobacco consumption, smoking less than 20 g tobacco each day gave the OR of 2.1 (95% CI: 0.98-4.4), and the OR when smoking more than 20 g tobacco per day was 2.5 (95% CI: 1.1-5.6). The OR of H. pylori infection was 5.0 (95% CI: 2.2-11.2). Among H. pylori-seropositive citizens, current smoking was associated with an increased risk of 2.3 (95% CI: 1.1-4.7) compared with non-smoking H. pylori-positive persons.'] Does tobacco smoking increase the risk for gastric adenocarcinoma among Helicobacter pylori-infected individuals?