The association between body mass index (BMI) and the incidence rate of breast cancer has been examined in 236 cases of breast cancer that developed among 23,826 Norwegian women during 11 to 14 years of follow-up. At the time of height and weight measurement they were 35 to 51 years of age, and at the end of follow-up their age was between 46 and 63 years. There was an overall age-adjusted incidence rate ratio (IRR) of 0.52 (95% confidence limits, 0.34 and 0.77) for women in the highest quartile of BMI compared to women in the lowest quartile, which was confined to an effect observed among women who were diagnosed at age 50 or earlier (IRR = 0.36). The association with BMI displayed an inverse dose-related trend (X2 for trend = 14.22, p < 0.001). The negative trend was particularly pronounced among non-smoking women (X2 = 14.63), and no clear trend associated with BMI was observed among women who smoked 10 or more cigarettes per day (X1 = 0.41), indicating an interaction between BMI and cigarette smoking (X2 interaction = 3.86, p = 0.05). We thus suggest that there is a negative association between body mass index and risk of breast cancer among premenopausal women.