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Cohort profile: the Danish nurse cohort.

From the 1970s to the mid-1990s, 20–30% of Danish women between 55 and 65 years of age used hormone replacement therapy (HRT). It was primarily used to alleviate climacteric symptoms, but from 1980s the therapy was also used for prevention of osteoporosis and from the 1990s for prevention of cardiovascular diseases. The literature based on observational studies, primarily US studies, suggested a considerable risk reduction of osteoporotic fractures and coronary heart disease, whereas findings on the effect of HRT on breast cancer were inconsistent. In 1976, the US Nurses’ Health Study was established, comprising 121 700 female registered nurses, who were between 30 and 55 years of age. This cohort was originally established to assess risk factors for cancer and cardiovascular diseases, with a particular interest in effects of oral contraceptives, use of hormones, cigarette smoking and other lifestyle factors. Ever since, the participants have been followed every other year with a mailed questionnaire to update exposure information and information on illnesses diagnosed since the previous questionnaire. Inspired by this US cohort of nurses, the Danish Nurse Cohort was established to examine the benefits and risks associated with HRT use in a European population.