Obesity, overweight, and a sedentary lifestyle-all common conditions in breast cancer patients-are likely to be associated with poor survival and poor quality of life in women with breast cancer. Diet-related factors are thought to account for about 30% of cancers in developed countries. Most studies of diet and healthcare have focused on the role of single nutrients, foods, or food groups in disease prevention or promotion. Recent cancer guidelines on nutrition and physical activity emphasize diets that promote maintenance of a healthy body weight and a prudent dietary pattern that is low in red and processed meats and high in a variety of vegetables, fruits, and whole grains. Except for dietary fat, few nutritional factors in adult life have been associated with breast cancer. Extensive data from animal model research, international correlations linking fat intake and breast cancer rates, and case-control studies support the hypothesis that a high-fat diet is conducive to the development of breast cancer in postmenopausal women. Conflicting findings from cohort studies, however, have created uncertainty over the role of dietary fat in breast cancer growth and recurrence. Results from large-scale nutritional intervention trials are expected to resolve such issues. As new and improved data on dietary factors and patterns accumulate, dietary guidelines for cancer risk reduction will become more focused.