We studied an outbreak of A(H1N1) influenza in an Israeli military unit of 336 healthy young men to determine the relation of cigarette smoking to the incidence of clinically apparent influenza and to the influenza-antibody response. Of 168 smokers, 68.5 per cent had influenza, as compared with 47.2 per cent of nonsmokers (P less than 0.0001). Influenza was also more severe in the smokers; 50.6 per cent of the smokers lost work days or required bed rest, or both, as compared with 30.1 per cent of the nonsmokers. The proportion of all influenza in smokers that was attributable to smoking was 31.2 per cent (95 per cent confidence intervals, 16.5 to 43.1 per cent). For severe influenza, the attributable risk in the smokers was 40.6 per cent (95 per cent confidence intervals, 21.6 to 54.8 per cent). A quarter of all severe morbidity from influenza in the overall study population was attributable to smoking. Antibody levels to A/USSR/90/77(H1N1) antigen were higher in smokers but not markedly so. We conclude that smoking is a major determinant of morbidity in epidemic influenza and may contribute substantially to incapacitation in outbreaks in populations that smoke heavily.