This is a randomized, phase III trial of a 12 week exercise program, followed by a 36 week voluntary, self-directed exercise program (total 48 weeks) compared to standard supportive care. The purpose of the trial is to determine whether the exercise program is better than standard supportive care to decrease the muscle and joint aches (musculoskeletal symptoms) that result from aromatase inhibitors.

Hypothesis An exercise program that includes stretching, strengthening and aerobic components may improve physical quality of life among women with musculoskeletal symptoms who are taking aromatase inhibitors.

Inclusion Criteria:

• Women who have been on an adjuvant AI for at least 6 weeks.
• The presence of some musculoskeletal symptoms.
• Eastern Cooperative Oncology Group (ECOG) performance status 0, 1
• ER and or PgR positive early breast cancer, stages I, II or III
• Completion of any adjuvant chemotherapy (if given) and primary breast cancer surgery.
• Be willing to switch to Anastrozole if on another AI
• Signed written informed consent

Exclusion Criteria:

• Metastatic disease (no imaging or labs required in absence of clinical suspicion of metastases)
• Evidence or history of inflammatory arthropathy (for example, rheumatoid arthritis)
• Any medical condition (for example, significant heart disease) that would prevent a subject from completing an exercise program.
• Unable to take Aromatase Inhibitor (AI)
• Planned secondary reconstruction procedures during the 48 week study period
• Any medical, physical, geographic, emotional, or economic condition that in the judgement of the clinician/study doctor, would make an individual unable or unlikely to be able to exercise three times a week for 12 weeks.
• Unable to complete a study log book and questionnaires (unable to read and write English and no one at home who can adequately do so).
• The use of any exogeneous estrogen and/or progesterone except Estring and natural plant estrogens