This randomized trial aims to assess if a well defined subgroup of breast cancer patients benefit from a controlled physical activity program.

All patients will be screened for their physical ability to complete the interventional exercise program. Only patients capable and willing to take part in this study and meeting all inclusion and exclusion criteria will be randomized.

All patients are to receive identical counseling for ideal nutritional, life-style and physical activity.

Patients randomized to Arm 2 will additionally undergo a controlled and observed program of physical activity for a period of 6 months. Thereafter the patients are expected to adhere to a comparable, unobserved exercise program at home.

Inclusion Criteria:

• Signed informed consent
• Postmenopausal women with hormone receptor positive breast cancer (as defined by HR >= ++ and/or PR >= ++) who are treated with aromatase inhibitors
• ECOG performance status <= 2
• all age groups

Exclusion Criteria:

• Pathologic ergometry
• Uncontrolled diabetes mellitus
• Uncontrolled hypertension
• COPD > II
• Active opportunistic infection
• NYHA heart failure III or IV