Physical activity is an important stimulus in the prevention of bone involution. Bilateral bone mineral mass and width of the radius, ulna, and humerus were measured by single photon absorptiometry on 200 women between the ages of 35 and 65 over a 3–4 year period. Two groups were formed: 80 in a control group and 120 in a physical activity group exercising 45 min/day, 3 days/week. The control and physical activity groups were similar in age, height, weight, and level of physical fitness at the beginning of the study. The control group lost bone mineral mass at a rate similar to that of the general population. The left radius bone mineral declined 2.44% per year. Similar rates of decline were observed in the other bones measured. The physical activity group increased in fitness by 13% in the first year of the study. The bone mineral mass of the exercise group declined the first year and then increased; thereby the bone mineral data for the exercise group was analyzed in two sets. During the first year of the program, a decline of 3.77% in bone mineral mass of the left radius, a significantly greater loss than that of the control group, was observed. Decline rates were similar in the other bones measured. In the second and third years, bone mineral mass of the left radius increased by 1.39% per year, and the rate of change was significantly different from that of the control group. There were similar increases in the other bones measured. The data from this study support the importance of physical activity in the prevention of bone mineral loss in the aging female.