A prospective study of breast-cancer risk in postmenopausal women has been carried out with the cooperation of 50 general practitioners. A total of 7,259 women have been followed up for an average period of 5.4 years (maximum 8.1 years); 70 cases of breast cancer occurred. The main finding was a pronounced effect of body weight and height on breast cancer risk; the significance of this in explaining international differences in incidence is stressed. High parity counteracted the risk of high weight regarding breastcancer. Single women did not show a high risk because of their relatively low body weight. The relative risk in women with a previous mastectomy was five-fold.