Analyses of dairy consumption and breast cancer incidence have yielded conflicting results. In this prospective cohort study of 48,844 premenopausal Norwegian women, we examined the relationship between childhood and adult milk consumption and breast cancer incidence. During a mean follow-up time of 6.2 years, 317 incident cases of breast cancer were diagnosed. Information on childhood and adult milk consumption was obtained from frequency questions mailed to the participants in 1991–92. Milk consumption as a child was negatively associated with subsequent breast cancer among the youngest women (34–39 years) (p for trend = 0.001), but not among older ones (40–49 years). Adult milk consumption tended to be negatively related to breast cancer incidence (p for trend = 0.12) after adjustment for age, reproductive and hormonal factors, body mass index, education, physical activity, and alcohol consumption. Women drinking more than 3 glasses of milk per day had an incidence rate ratio of breast cancer of 0.56 (95% confidence interval 0.31–1.01) compared with women not drinking milk. Analyses according to type of milk consumed and milk fat consumption did not reveal any clear associations. A combination of childhood and adult milk consumption produced a clear negative trend in breast cancer incidence rate ratios with increasing milk consumption (p = 0.03). © 2001 Wiley-Liss, Inc.