• Disease-free survival [ Time Frame: 8 years ] [ Designated as safety issue: No ]
  

• Overall survival [ Time Frame: 8 years ] [ Designated as safety issue: No ]
  
• Distant disease-free survival [ Time Frame: 8 years ] [ Designated as safety issue: No ]
  
• Weight at 6, 12, 24, 36, 48 and 60 months compared to baseline [ Time Frame: 5 years ] [ Designated as safety issue: No ]
  
• Health-related quality of life at 6, 12, 24, 36, 48 and 60 months compared to baseline [ Time Frame: 5 years ] [ Designated as safety issue: No ]
  
• Other medical endpoints such as diabetes, cardiovascular disease, arthritis, fasting biomarkers at 12 months compared to baseline (for women participating in a biospecimen substudy) [ Time Frame: 8 years ] [ Designated as safety issue: No ]
  
• Compliance with study procedures. [ Time Frame: 8 years ] [ Designated as safety issue: No ]
  

The primary objective of this trial is to evaluate the effect of the addition of a 2 year, centrally delivered individualized, telephone-based lifestyle intervention focusing on weight management to a mailed educational intervention on Disease Free Survival in post-menopausal women with early stage breast cancer (hormone receptor positive), BMI ≥24-<40 kg/m2, who are receiving standard letrozole adjuvant therapy. The telephone intervention will involve 19 phone calls, as well as mailings and a participant manual; women will be asked to lose up to 10% of their weight by reducing their caloric and fat intake (by 500-1000 kcal/day, 20% calories fat) and increasing their moderate physical activity (to 150-200 minutes/week). Secondary outcomes include overall survival, distant disease-free survival, weight change, health-related quality of life (HRQOL), selected non-cancer medical events and biologic factors (notably insulin). Approximately 2,150 women will be enrolled; follow-up will continue until target event rates have been met (anticipated 4-6 years after completion of the intervention). This sample size will provide 80% power (type 1 error 0.05 2-tailed) to detect a hazard ratio (HR) for DFS of 0.74-0.76 in the weight loss intervention arm.

Inclusion Criteria:

• Post-menopausal woman with a diagnosis of invasive breast cancer (T1-3,pN0-3,M0) for which definitive surgery was performed during the previous 15 months.
• On adjuvant hormonal therapy with letrozole at study entry.
• BMI ≥ 24 kg/m2 and < 40 kg/m2.
• Medical, surgical or radiation oncologist willing to agree 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.