Many smokers are unable or unwilling to completely quit smoking. A proposed means of harm reduction is to reduce the number of cigarettes smoked per day. However, it is not clear whether this strategy decreases the risk for tobacco-related diseases. To assess the effects of smoking reduction on lung cancer incidence. Observational population-based cohort study with up to 31 years of follow-up from the Copenhagen Centre for Prospective Population Studies, which administrates data from 3 longitudinal studies conducted in Copenhagen and suburbs, the Copenhagen City Heart Study, the Copenhagen Male Study, and the Glostrup Population Studies, Denmark. Participants were 11,151 men and 8563 women (N = 19,714) aged 20 to 93 years, who attended 2 consecutive examinations with a 5- to 10-year interval between 1964 and 1988. Participants underwent a physical examination and completed self-filled questionnaires about lifestyle habits. The study population was divided into 6 groups according to smoking habits: continued heavy smokers (> or =15 cigarettes/d), reducers (reduced from > or =15 cigarettes/d by minimum of 50% without quitting), continued light smokers (1-14 cigarettes/d), quitters (stopped between first and second examination), stable ex-smokers, and never smokers. Incident primary lung cancer cases assessed by record linkage with the National Cancer Registry until December 31, 2003. There were 864 incident lung cancers during follow-up. Using Cox regression, the adjusted hazard ratio (HR) for lung cancer in reducers was 0.73 (95% confidence interval [CI], 0.54-0.98) compared with persistent heavy smokers. The HR for light smokers was 0.44 (95% CI, 0.35-0.56); for quitters, HR 0.50 (95% CI, 0.36-0.69), for stable ex-smokers, HR 0.17 (95% CI, 0.13-0.23), and for never smokers, HR 0.09 (95% CI, 0.06-0.13). Among individuals who smoke 15 or more cigarettes per day, smoking reduction by 50% significantly reduces the risk of lung cancer.