The height and weight of 570,000 Norwegian women, aged 30-69 years, were measured and the subjects were then followed up for 6-18 years with regard to breast cancer morbidity and mortality. They were arranged in 5-year age-groups. In all age-groups the tallest women were found to have the highest risk for both morbidity and mortality. Overweight was a risk factor for breast cancer mortality in all age-groups, but it was a risk factor only in the post-menopausal age-groups in the case of morbidity. It appeared to protect against breast cancer in the pre-menopausal age-group. Stages I and II—IV follow-up endpoints show negative and positive associations, respectively, with overweight. It is not likely that this can be fully explained by earlier detection of cancer among slim women.