The purpose of this cohort study with up to 31 years of follow-up of 19,714 persons from three Danish population-based studies was to analyse the association between smoking reduction and lung cancer risk. Information on smoking habits and changes in these were linked to the Danish Cancer Registry for incident lung cancers and analysed with Cox regression. Results showed that compared to sustained heavy smokers, smoking reduction by at least 50% reduced lung cancer incidence by 27%. We conclude that smoking reduction is associated with a significantly reduced risk of lung cancer.