STUDY OBJECTIVE: To determine the relation between cigarette smoking and upper respiratory infection or viral syndrome. DESIGN: A prospective cohort study of soldiers during 13 weeks of basic combat training. Each soldier received a smoking questionnaire before the beginning and at the conclusion of basic combat training. The incidence of respiratory illness among the recruits was evaluated. SETTING: Fort Benning, Georgia. PARTICIPANTS: 1230 soldiers met the criteria for inclusion and completed the study. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: According to the definition of smoking status in the initial questionnaire, smokers had a relative risk for upper respiratory infection of 1.46 (95% CI, 1.1 to 1.8). This effect was not substantially altered by different definitions of smoking status. CONCLUSIONS: Young military recruits who smoked during basic combat training had more upper respiratory infections than nonsmokers. Cigarette smoking can substantially increase the risk for upper respiratory infection in young men.