The effect of smoking on breast cancer risk was evaluated in a population-based case-control study, including 1,480 women diagnosed with breast cancer in Denmark between 1983-84. They were identified from the files of the nationwide clinical trial of the Danish Breast Cancer Cooperative Group and the Danish Cancer Registry. The control group was an age-stratified sample of 1,332 women from the general population. Data on risk factors were collected by self-administered questionnaires. The risk of breast cancer among current smokers and ex-smokers was similar to that in non-smokers, both risk estimates being close to unity. No dose-response relation was observed for any measure of smoking (age at start, duration, number of cigarettes per day, or cigarette-years of exposure) in all subjects, and when pre- and post-menopausal women were examined separately. These findings suggest that smoking is not associated with the risk of breast cancer.