Study of the humeral cortex of 89 acute cadavers showed that an important factor contributing to the physiologic bone loss of aging is increasing bone porosity. Mean cortical porosity increases in both sexes with age, from 4.6% in men and 4% in women at 40 years of age to 10% and more at age 80. In the population studied, no significant difference of porosity was observed between men and women. Apparent mineral density is linked to porosity, and decreases markedly with age in women. Changes in men are lesser in magnitude and show a larger difference of density values. Correction of the apparent mineral density, by a factor reflecting the proportion of vascular and resorption spaces in the cortical bone, produces a true mineral density which does not vary significantly with age in either sex. The density values obtained for the proximal humerus differ from those in the literature which represent the femur. However, they are more readily compared with the results of clinical densitometry and may have greater clinical applications.