The photon absorption technique was used to measure bone mineral content in the radius repeatedly with time to determine the feasibility of using this method in studies on the rate of loss with aging. Two groups of postmenopausal women were followed, a younger group for an average of 3.74 years and an older group for an average of 2.37 years. It was determined that estimates of rate over periods less than two years were too low in precision to be useful. There were significant differences in rates among individuals. The loss of mineral from the midshaft site was determined to be linear within the precision of the method and over an average interval of 3.74 years. From these data the size of groups and length of time needed for a study to determine the effect of a treatment upon inhibition of bone loss was predicted. When the inhibition is 50 per cent of the control rate, it would be possible to detect a significant (alpha=0.05) inhibition with a certainty of 95 per cent (1-beta = 0.95), if 56 subjects were followed in each group for a period averaging 2.70 years. A more potent inhibition or more frequent visits would reduce the number of subjects and duration of study needed. The separation of individuals into groups of significantly more rapid and slower rates of loss was demonstrated. From these data, it is possible to predict that one-third of a sample could be so characterized after 3.5 years. The photon absorption technique can be applied to the measurement of the rate of loss of mineral with aging with a precision that is sufficicient for certain types of clinical investigations.