BACKGROUND: Physical activity may affect hormonal concentrations that may have bearing on the risk of breast cancer. The aim of this study was to investigate whether physical activity affects this risk. MATERIAL AND METHODS: All women aged 51-60 invited over an eight-week period in 1997 to a pilot breast cancer screening in Norway were also invited to participate in this study (n = 10,513). Information was collected by questionnaire about intensity and duration of physical activity from the age of ten up to the present, in five-year intervals. The activity was converted to scores of Metabolic Equivalent Task (MET). RESULTS: 74% og those invited attended the screening; 76% og those attending also took part in this study. A total of 2,300 of women gave adequate information about physical activity; only 12 of them had breast cancer. Those with cancer reported less physical activity compared to the controls. Physical activity corresponding to > or = 15 MET/week at age 10-24 reduced the risk of breast cancer at age 51-60 significantly (OR = 0.2; CI 0.0-0.9). INTERPRETATION: The small number of breast cancer cases limits the validity of the results, though the data indicate that physical activity of > or = 15 MET/week at age 10-24 may reduce the risk of breast cancer at age 51-60.