1. A detailed study of density variations with age in cortical bone samples from different areas of the femoral diaphysis has been carried out.

2. Bone of relatively high density and, conversely, of low density was found to have a spiral pattern along the bone shaft.

3. Moving distally along the femoral shaft there was a transposition of the greatest density from the anterior to the posterior aspect.

4. In the erect body posture the femoral inclination is such that areas of denser bone might be expected to be aligned vertically because of the weight-bearing characteristics of the femur.

5. Most areas of the cortex tended to decrease in density after the age of fifty, the less dense areas changing more than those with an initial high density. Thus, osteoporosis tended not only to maintain but to accentuate the spiral pattern of density distribution by increasing the difference between dense and less dense bone.

6. In the distal region of the diaphysis bone resorption was greatest anteriorly but hardly affected the posterior aspect. A densitometric comparison between these two sites provided a clear indication of the effect of osteoporosis.

1. Discs of bone from two fixed sites on the front of the femur were taken from ninety-one necropsy subjects and the density, width and histological appearance of the cortical bone were examined.

2. Cortical thickness, and the density of the femoralcortex, decreased with increasingage of bone.

3. There was an increase in the rate of resorption of the bone cortex from both sexes after the fifth decade.

4. The difference between the density of the metaphysial cortex and that from the diaphysis increased for both sexes after the age of fifty, because of the greater metaphysial resorption.

5. There was no change in the degree of mineralisation of the cortical bone with age. The decrease in density with age is, therefore, accounted for by resorption.