All women (n = 11,809) aged 44-53 years from a representative district of Denmark were asked by questionnaire about their date of birth, date of latest menstrual bleeding, possible gynaecological operations, smoking habits, and use of medications. A total of 9,411 (80%) questionnaires were returned. After exclusions because of incomplete information (n = 275), possible surgical menopause (n = 1,270), and treatment with gonadal hormones (n = 2,221), 5,645 women remained suitable for to study. The reproducibility of the information given by these women was excellent as judged by personal interviews of 315 participants. The study population was divided into five 2-year age groups, and the proportion of postmenopausal women among non-smokers and heavy smokers was calculated. Differences in respect to the menopausal status were not observable in the oldest and the youngest two age groups comprising just a few per cent of post-menopausal women. However, heavy smokers in the other two age groups had passed the menopause earlier than the non-smokers (p less than 0.001). This finding suggests cigarette smoking as a promotor of the menopause.