Background. Mortality from smoking-related diseases in women is increasing worldwide. Studies comparing hazards associated with smoking in women and men based on a sufficient number of heavy smokers of both genders are lacking. Methods. We used pooled data from three prospective population studies conducted in Copenhagen to compare total and cause-specific mortality in relation to smoking habits. A total of 30,917 subjects, 44% women, with initial. examinations between 1964 and 1992 were followed until 1994 for date and cause of death. Results. During follow up, 2900 women and 5744 men died. Smoking characteristics differed considerably with gender, particularly in the older subjects. Overall mortality rates in smokers were approximately twice those in people who never smoked. Positive associations with smoking in both men and women were confirmed for all-cause mortality as well as mortality from respiratory disease, vascular disease, lung cancer, and other tobacco-related cancers. Despite large gender differences in age at smoking debut, total and cause-specific relative mortality in smokers was similar in men and women. After excluding non-inhalers, relative risks associated with smoking for respiratory and vascular disease were higher for women than men whereas there were no gender differences in smoking related risk of cancers. Conclusions. The relative risks suggest that women may be more sensitive than men to some of the deleterious effects of smoking. However, because of lower baseline mortality rates in women, rate differences may be similar and results should be interpreted with caution.