In this prospective study, the relationship between energy and fat consumption and the risk of breast cancer was examined. Between 1977 and 1983, 31,209 Norwegian women, 20 to 54 years of age attended a health screening. The attendees were given a food-frequency questionnaire to be completed at home, and this was returned by 25,892 (83%). During the 7 to 13 years of follow-up, 248 cases of breast cancer were identified for analysis by linkage to the Norwegian Cancer Registry. The relative risk of women who ate meat more than 5 times a week, as compared with those who consumed meat twice or less than twice a week, was 2.44. Consumers of 0.75 litres or more of full-fat milk daily had a relative risk of 2.91 compared with those who consumed 0.15 litres or less. A positive relationship was found between those reporting the highest quartile of mono-unsaturated fat consumption and the risk of breast cancer. The main foods contributing to the mono-unsaturated fat index were edible fats, meat and milk.