Lifestyle Intervention Study in Adjuvant Treatment of Early Breast Cancer

Inclusion Criteria: * Post-menopausal woman with a diagnosis of invasive breast cancer (T1-3,pN0-2,M0) for which definitive surgery was performed during the previous 36 months. * On adjuvant hormonal therapy with letrozole at time of randomization (either as initial adjuvant hormonal therapy or after a switch from tamoxifen or other hormonal therapy). * BMI ≥ 24 kg/m2. * Medical, surgical or radiation oncologist agrees to subject participation in a diet and physical activity program. Exclusion Criteria: * Life expectancy less than five years. * Self-reported inability to walk at least 2 blocks (at any pace). * Insulin requiring diabetes (telephone directed diet and physical activity changes would be difficult in this population without close coordination with the treating physician). Non-insulin requiring diabetics are eligible for the study. * Serious digestive and/or absorptive problems, including inflammatory bowel disease and chronic diarrhea that preclude adherence to the study diet. * Cardiovascular, respiratory or musculoskeletal disease or joint problems that preclude moderate physical activity. Moderate arthritis that does not preclude physical activity is not a reason for ineligibility. * Psychiatric disorders or conditions that, in the opinion of the investigator, would preclude participation in the study intervention (e.g. untreated major depression or psychosis, substance abuse, severe personality disorder). * Patients on aromatase inhibitors other than letrozole at study entry. * Known recurrence of breast cancer (local, regional or distant) at any time prior to study entry. * History of other malignancies except: adequately treated non-melanoma skin cancer, curatively treated in situ carcinoma of the cervix, or other solid tumours curatively treated with no evidence of disease for ≥ 5 years. Prior in situ cancer of the breast is not a reason for exclusion. * Patients not fluent in either English or French (spoken and written). * Patient unwilling or unable to provide informed consent.