Risk factors for breast cancer and their prognostic significance.

Since 1960 a 50% increase has occurred in breast cancer incidence in Denmark. With the present rates one out of fifteen Danish women will develop breast cancer before the age of 75 years. To evaluate the influence of known and suspected epidemiologic factors on the risk of developing breast cancer and on prognosis, a population-based study has been carried out, where cases were identified from the files of the Danish Breast Cancer Cooperative Group (DBCG) and the Danish Cancer Registry. Results are communicated from the case-control part of this study, including 1486 breast cancer cases aged less than 70, diagnosed over a one-year period, and an age-stratified-random sample of 1336 women without breast cancer from the general population. Data on risk factors were collected by self-administered (mailed) questionnaires. Classical risk factors, such as high social status, nulliparity, early menarche and late natural menopause were confirmed, while no association was found between breast cancer and maternal age at first childbirth.