The main objective of this study is to investigate the possible impact of a low fat diet on the incidence of breast cancer in high risk women when compared to a similar female population consuming a normal diet. To achieve this goal, is necessary to demonstrate that (1) long-term behavior modification and compliance with respect to dietary fat intake is possible without deleterious effects on the patient and family; (2) a five-year study period is sufficient to demonstrate a difference in breast cancer incidence; (3) the study can be conducted without exposing the intervention group to undue risks of protein-calorie undernutrition and other nutritional derangements. We propose to conduct this investigation through a randomized prospective clinical trial enlisting the voluntary involvement of 300 women from a group of 727 who have been identified to be at high risk of developing breast cancer and who are or have been participants of a well-established women's cancer control program or its parent, a successful breast cancer detection demonstration project with more than 10,000 participants and follow-up of 100%. The detailed methodology, instruction and monitoring of dietary intervention has been developed and will be directed in strict compliance with the final protocol by a successful nutritional support team that is exclusively responsible for nutritional surveillance and support of all patients in an institution dedicated to the care of the cancer patient.