We examined the subchondral bone architecture of the femoral head in relation to trabecular microfracture. Three groups of femoral head specimens were studied. Twenty-eight specimens taken during hip replacement had grade III or IV arthrosis (70 8 years). From autopsy, 40 femoral heads were obtained, 18 in a group greater than 50 years of age (72 10 years) and 22 in a group less than 50 years of age (25 11 years). None of these 40 heads had worse than grade II arthrosis. Coronal slices of the femoral heads were macerated and examined under a dissecting microscope to count trabecular microfractures. For bone histomorphometry, blocks were taken from the subchondral principal compressive and tensile trabeculae. The bone volume, trabecular thickness, and marrow space were quantified. In the subchondral principal compressive region, the arthrotic group had more bone volume, thicker trabeculae, similar trabecular space, and trabecular microfractures when compared with the group greater than 50 years old. In the tensile region, there were no differences except for decreased trabecular microfracture number in the arthrotic group. With the thinnest trabeculae in the compressive region occurring in the greater than 50 years old group, the trabeculae of the younger age group have thinned with age, but with the onset of arthrosis, the thinning is overtaken by pathologic thickening of trabeculae.