In Denmark, incidence of female breast cancer remained constant from 1943 to around 1960, whereafter a steady increase has occurred, the level today being about 50% higher than in 1960. No equivalent rise has been observed for breast cancer mortality. Influence of hormonal and dietary factors on breast cancer risk and survival was evaluated in a combined population-based case-control and follow-up study, including 2,445 women, aged less than 70 years, diagnosed with breast cancer in Denmark between 1 March 1983 and 31 August 1984, identified from the files of the nation-wide clinical trial of the Danish Breast Cancer Co-operative Group (DBCG) and the Danish Cancer Registry. The control group was an age-stratified random sample of the general female population, selected from the Central Population Register. Data on risk factors were collected by self-administered questionnaires. Clinical and pathological tumour characteristics derived from DBCG. The case-control analysis confirmed an overall increased risk of breast cancer associated with urban residence, high social status, nulliparity, early age at menarche, late age at natural menopause, hormonal replacement therapy, high dietary fat intake, and high alcohol consumption in a subgroup. It failed to detect an association with age at first childbirth, oral contraceptives, smoking, intake of vegetables, tea, coffee, and sweeteners. Survival was determined by tumour size, skin invasion, number of positive lymph nodes, and grade. There was no relation between survival and reproductive or hormonal factors, dietary variables, alcohol consumption, or smoking. However, a complex relationship may exist between survival and body mass index.