The goals of the study are to examine the effect in postmenopausal women of a one-year moderate intensity exercise intervention, as compared with a sedentary pattern (low-level stretching program), on: serum estrogens, androgens, sex hormone binding globulin, insulin, triglycerides, glucose, aromatase, FSH, LH, as well as parameters of body fat mass.

Inclusion Criteria:

• Ages 55-75 years at first group meeting
• Physically able to undertake a moderate exercise program
• Sedentary activity pattern (exercising < 2 times/week for 20 minutes at a level to produce sweating)
• Postmenopausal (not experiencing menstrual periods for the previous 12 months or FSH >30 mIU/ml; for women who have had a hysterectomy, an FHS >30 mIU/ml will classify them as postmenopausal)
• Body Mass Index (BMI) greater than 25.0
• Able to follow directions and fill out questionnaires, and exercise diaries and logs in English
• Agrees to be randomly assigned to either the exercise intervention or control group
• Gives informed consent to participate in all screening and study activities
• Able to come to the FHCRC for clinic visits and classes, to the UW Medical Center for CT scans, and to the UW NETT for measurements and instruction

Exclusion Criteria:

• Plans to leave Western Washington within the follow-up period
• Using hormone replacement, tamoxifen, or oral contraceptives in past 6 months
• Currently having severe hot flashes (that might prompt participant to start hormone replacement therapy before the end of study)
• Current unstable thyroid disease or unstable dose of thyroid medications or TSH >5.0 IU/ml or <0.5 IU/ml
• Invasive cancer in the past 10 years (except nonmelanotic skin cancer)
• Any other endocrinologic abnormality
• Breast, endometrial, ovarian, or melanoma cancer or carcinoma-in-situ at any time
• Morbidly obese (BMI > 40)
• Current or planned use of corticosteroids
• Currently on or planning to enter a structured weight loss program or to take diet drugs
• Current use of certain other medications, such as any likely to interfere with adherence to an exercise program or likely to interfere with study outcomes
• Moderate to high alcohol intake (more than 2 drinks per day)
• Current smoker
• Diabetes mellitus (fasting hyperglycemia)
• Abnormal screening labs (hematocrit < 32 or > 48, white blood cells < 3.0 or > 15.0, potassium < 3.5 or > 5.0, fasting glucose > 140, creatinine > 2.0), or abnormalities on screening physical judged by study physicians to contraindicate participation in an exercise program
• Contraindications for entry into a training program118 including: unstable angina, uncontrolled hypertension (systolic > 200, diastolic > 100), orthostatic hypotension (> 20), moderate/severe aortic stenosis, uncontrolled arrhythmia, uncontrolled congestive heart failure, third degree heart block, pericarditis, myocarditis, pulmonary/systemic embolism within the past 6 months, thrombophlebitis, ST displacement > 3 mm at rest, history of cardiac arrest)
• Contraindications for exercise testing118 including: recent (within 6 months) cardiac event (MI, pulmonary edema, myocarditis, pericarditis), unstable angina, uncontrolled arrhythmia, third degree heart block, left bundle branch block, acute congestive failure, recent (within 6 months) pulmonary/systemic embolus, severe electrolyte abnormality
• Meets criteria to stop screening exercise test118 as decided by overseeing physician (Dr. Schwartz or McTiernan)
• volunteering in order to lose weight
• alcohol or drug abuse, significant mental illness (as assessed by study staff impression)