OBJECTIVES To determine the effects of up to 14 months of aerobic exercise on measures of bone density in older adults. DESIGN Randomized controlled trial with subjects assigned to either an aerobic exercise condition, non-aerobic yoga, or a wait list non-exercise control group for 4 months. Aerobic fitness and bone density were evaluated in all subjects at baseline (Time 1) and after 4 months (Time 2). A semi-crossover design was utilized with all subjects completing 4 months of aerobic exercise, followed by another evaluation (Time 3). All subjects were then given the option of 6 additional months of aerobic exercise, after which they had a fourth evaluation (Time 4). SETTING An outpatient exercise rehabilitation facility at a large, major medical center. SUBJECTS One-hundred-one healthy men (n = 50) and women (n = 51) over age 60 (Mean age = 67.0), recruited from the community. INTERVENTION The exercise program included stretching, cycle ergometry, and walking three times per week for 60 minutes throughout the course of the study. OUTCOME MEASURES Aerobic fitness (VO2max) as assessed by cycle ergometry, and bone density (bone mineral content) measured by single photon absorptiometry. RESULTS Subjects achieved a 10%-15% increase in VO2max after 4 months of exercise training, and 1%-6% further improvement with additional training. Aerobic fitness was associated with significant increases in bone density in men, but not women, who maintained aerobic exercise for 14 months.