The observation of large differences in breast cancer rates between countries has led to the hypothesis that excessive intake of dietary fat is an important risk factor for breast cancer in women. Case-control and prospective studies, however, generally have failed to show associations between dietary fat and breast cancer risk. There therefore is only weak evidence that modest reductions in fat intake (for instance to levels of 30% of caloric intake from fat) will reduce breast cancer risk. The possible benefits of lowering fat intake to levels substantially below 30% of calories will need to be tested in a randomized trial. In the meantime, the possible roles of micronutrient imbalances and childhood nutritional factors need to be studied better. Obesity is related to breast cancer in a complex way that suggests that a hormonal correlate of excessive body weight might affect breast cancer growth and metastasis. The potential benefit of intentional weight loss as an adjunct breast cancer treatment deserves further study. Many studies have suggested that drinking alcohol, even at modest levels, might increase breast cancer risk. Because the potential benefits of modest levels of alcohol for cardiovascular disease may outweigh the risk for breast cancer, recommendations for total alcohol abstinence may be premature for women with an average breast cancer risk. Women at unusually high risk for breast cancer who have a lower-than-average risk for cardiovascular disease, however, might make an informed decision to abstain from alcohol intake. Following current dietary advice to increase the amount of fruits, vegetables, and whole grains in the diet while reducing fats is certainly prudent for women to reduce their risk of several chronic disease, but current data points to the somber conclusion that such changes probably will have little effect on breast cancer risk.