To evaluate the influence of oral contraceptives (OCs) on breast cancer risk, a population-based case-control study was conducted in Denmark. The study population included women aged less than 40 years (203 cases, diagnosed between 1 March 1983 and 31 August 1984, and 212 controls) and women aged 40-59 years (856 cases, diagnosed between 1 March 1983 and 29 February 1984, and 779 controls). Cases were identified from the nationwide clinical trial of the Danish Breast Co-operative Group and the Danish Cancer Registry. The control groups were age-stratified random samples of the general female population. Data on OC use and breast cancer risk factors were collected by self-administered questionnaires. In both age groups, no significant association was detected between breast cancer risk and duration, age at start, latency or recency of OC use. However, the data provided some limited support that OCs containing 50 micrograms or more oestrogen may be more harmful than pills with a lower oestrogen dose.