We have devised a new method for measurement of final depth of erosion in cancellous bone with an intra-individual precision of 4.3% and applied it to determine the mechanism of continuing reduction in trabecular thickness after menopause. Mean erosion depth (SD) was 40.8 (2.0) μm in 10 healthy postmenopausal women and 41.4 (2.1) μm in 10 age-matched patients with postmenopausal osteoporosis; the difference was not statistically significant. In contrast, wall thickness, using a method based on density differences between new and old bone, was 39.5 (2.0) μm in the normal subjects and 35.3 (2.0) μm in the patients with osteoporosis (p < 0.0001). The balance per remodeling cycle (ΔBMU) was −1.34 (2.49) μm in the normal subjects and −6.11 (1.95) μm in the patients with osteoporosis. This difference was also highly significant (p < 0.001). Indirect estimations of erosion depth and ΔBMU, based on the fall in trabecular thickness from an assumed premenopausal value of 147 μm and the number of remodeling cycles accumulated since menopause, agreed closely with the measured values. Erosion depth measured by the Eriksen method also showed no significant difference between the two groups, but because the values were substantially higher ΔBMU was improbably high in both groups, did not differ significantly between groups, and was inconsistent with the observed difference in trabecular thickness. We conclude that (1) the more rapid continuing loss of cancellous bone in patients with postmenopausal osteoporosis than in age-matched control subjects is due entirely to a difference in wall thickness, not to a difference in erosion depth; and (2) defective recruitment and/or function of osteoblasts is the major cellular mechanism of trabecular thinning in patients with postmenopausal osteoporosis and probably also in normal subjects. We emphasize that these conclusions do not speak to the mechanism of complete removal of trabeculae in the early years after menopause.