One hundred forty sedentary older adults will be recruited for this study. Participants will be randomized to an aerobic exercise intervention group, or to a stretching and toning control group. Aerobic exercise sessions will be conducted three times a week for one year, beginning at a light to moderate intensity level over the first two months and progressing to a moderate to high level for the remainder of the year. The control group will meet on the same basis as the intervention group, and will participate in an organized program of stretching, limbering, and toning for the whole body that is specially designed for individuals 60 years of age and older.