Rationale: Smoking is a primary risk factor for chronic bronchitis, emphysema and COPD, but since not all smokers develop disease it has been suggested that some individuals may be more susceptible to exogenous factors, such as smoking, and that this susceptibility could be genetically determined. Objectives: The aim of the present study was to assess, in a population based sample of twins, (a) to what extent genetic factors contribute to the development of chronic bronchitis, including emphysema, taking gender into consideration and, (b) whether the genetic influences on chronic bronchitis, including emphysema, are separate from those for smoking behaviour. Methods: Disease cases and smoking habits were identified in 44 919 twins older than forty years from the Swedish Twin Registry. Disease was defined as self-reported chronic bronchitis or emphysema, or recurrent cough with phlegm. Individuals having smoked 10 pack years or more were defined as smokers. Univariate and bivariate structural equation models were used to estimate the heritability specific for chronic bronchitis and that in common with smoking. Measurements and Main Results: Heritability estimate for chronic bronchitis was a moderate 40% and only 14% of the genetic influences were shared with smoking. Conclusions: Genetic factors independent of those related to smoking habits play a role in the development of chronic bronchitis.