In order to evaluate the influence of sex hormones on breast cancer risk, a population-based case-control study was conducted in Denmark, including 1,486 cases diagnosed over a one-year period. These were identified from the files of the nation-wide clinical trial of the Danish Breast Cancer Cooperative Group and the Danish Cancer Registry. The control group was an age-stratified random sample of 1,336 women from the general population. Data on risk factors were collected by self-administered (mailed) questionnaires. The major findings included a trend (p = 0.001) toward decreasing risk with increasing age at menarche in pre-menopausal women, trends toward increasing risk with continued menstrual cycles after the age of 50 in pre- and post-menopausal women (p-values of 0.01 and 0.002 respectively), and a trend (p = 0.002) toward increasing risk with increasing duration of non-contraceptive sex hormone usage in post-menopausal women. Information on brand names made it possible to examine types of hormones used, which showed an RR of 1.36 (95% CI 0.98-1.87) for sequential therapy with oestrogen and progestagen and RR = 2.31 (95% CI 1.37-3.88) for combined oestrogen-androgen treatment. These results should be interpreted with caution, however, needing verification from other studies. No significant association was observed between breast cancer and self-reported height and weight.