BMC Public Health 2007, 7:149 http://www.biomedcentral.com/1471-2458/7/149 Background Early onset of menopause is a risk factor of osteoporosis [1] and death from ischemic heart disease [2], although it seems to protect against breast cancer [3,4]. These effects may result from an earlier decrease in the amount of female sex hormones [5,6]. Today, women tend to want children later and the age of giving birth is getting closer to the age of menopause. In Northern Europe, the mean age of the firstborn child is 27.1 years. In Oslo it is as high as 28.9 years, and 2.3% of Norwegian women have chil- dren after the age of 40 [7]. Early menopause is becoming an important public health issue as a risk factor for fertility problems. During the last two decades, several risk factors for early menopause have been investigated. The results are incon- sistent, but the trend suggests that nulliparous women have an earlier onset of menopause than do parous women [8,9]. A high educational level [10,11], being overweight [8-10], a moderate intake of alcohol [10,12] and a higher intake of coffee [9] seems to protect against early menopause. Current smoking around the onset of menopause is an established risk factor for early meno- pause [10,13-16], whereas previous smoking seems to matter less [8,11,13,14,16]. Some studies have reported a dose-response effect of smoking on early menopause [8,16,17], although other studies show no such effect [11,13]. Among the few published studies of passive smoking and ovarian function, Everson et al. found that both passive and active smoking are associated with early menopause [18], whereas Cooper et al. [13] and Cramer et al. [17] reported an association with active smoking, but not with passive smoking. Detailed information was collected in 2000–01 from 59– 60-year-old women from Oslo about their past and present smoking habits, coffee and alcohol use, educa- tional level and other factors relevant to health. The prev- alence of smoking, both active and passive, is high in this cohort. The objective of this study is to investigate the association between early menopause and active and pas- sive smoking including a possible dose-response relation- ship with active smoking and early menopause. One further main aim of this study is to investigate the associ- ation between early menopause and coffee and alcohol consumption. Methods The data for this analysis were obtained from The Oslo Health Study conducted in the city of Oslo from May 2000 to September 2001. The sampling and data collec- tion have been described in detail elsewhere [19]. All inhabitants registered with an address in Oslo and born in the years 1970, 1960, 1955, 1940–41 and 1924–25 were invited to participate in the study, which included a ques- tionnaire and a simple clinical examination. The Norwe- gian Data Inspectorate has approved The Oslo Health Study, the Regional Committee for Medical Research Eth- ics has evaluated it, and it has been conducted in full accordance with the World Medical Association Declara- tion of Helsinki. Data from women born in 1940–41 were analysed for the present study. Of 4116 invited women in this group, 2357 participated (57.3%), all with full written consent. We excluded 159 women because of missing relevant infor- mation, most often on menopausal age and in some cases on the age of menarche. Another 75 were excluded because they reported a menopausal age of less than 40 years, as it is likely that a large percentage had undergone surgical menopause. The total number of participants was 2123, 51.6% of the total population in the age group. The participants were asked whether they were current smokers, former smokers or never smokers. The current and former smokers reported the number of cigarettes they smoked per day and at which age they started smok- ing on a daily basis. The former smokers also reported the number of years since they stopped smoking. With the help of the latter variable and the self-reported age of menopause, we estimated their smoking status at the age of menopause and during the years before. Total smoking exposure was estimated by multiplying the number of cig- arettes smoked daily by 365 and then by the number of years of active smoking. Participants were divided into three categories of active exposure to tobacco smoking: low (365–64,240 cigarettes), medium (64,241–158,775 cigarettes) and high (158,776–657,000 cigarettes) expo- sure. Passive smoking was recorded by asking participants: (1) how many hours a day was currently spent in a room where people smoke, (2) whether they had a smoking mother or father while growing up and (3) whether after the age of 20 they had lived in a household with anybody who smoked. Never-smokers who regularly spent time in the same room as smokers or answered yes to one of these two last questions, were defined as passive smokers. Participants were divided into three categories based on the reported age when the menses had stopped: early menopause (40–44 years), median menopause (45–54 years) and late menopause (>54 years). The participants were not asked about the cause of menopause – that is, if the menopause was natural or surgical or due to other medical treatment. The participants' educational level was divided into four levels: primary school, junior secondary school, senior secondary school or university education. 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