The relationship between nutritional factors including eating habits and the presence or absence of fibrocystic breast disease (FBD) with regard to other known risk factors has been investigated in women participating in the National Breast Screening Study (NBSS) in Montreal, Canada. Included in this case-control study were 334 patients with FBD aged 40-59 years at entry to NBSS and 340 controls. Personal and family history was collected from medical records and by interview conducted by a research nurse. Nutritional assessment was done by 24-hour dietary recall, by a food frequency questionnaire and by identification of changes in eating habits 5 years prior to diagnosis. Women > or = 50 years with FBD had higher intake of energy, carbohydrate, fibre, vitamin D, free folacin, calcium, sodium, potassium and magnesium but lower intake of cholesterol than controls. In women < 50 years body mass index and lower protein, niacin and zinc intake, were associated with FBD. In logistic regression analysis, breast pain and education > 12 years were among the most important predictive variables for FBD. The use of oral contraceptives, cholesterol intake > or = 300 mg/day, the proportion of energy provided by fat and saturated fatty acid intake above 10% of energy had a protective effect. The comparison of FBD patients diagnosed by biopsy and those by clinical examination indicated that increasing age had a significant association with the risk of biopsy and a rather weak association with the percentage of energy supplied by fat.