We compared indices of three-dimensional microstructure of iliac trabecular bone between 26 patients with vertebral compression fractures due to postmenopausal osteoporosis and 24 control subjects without vertebral fracture, who were matched for age, sex, race, menopausal status, and several densitometric and histologic indices of both cortical and trabecular bone mass. The patients with fracture had a significantly lower mean value (1.03±0.15 vs. 1.26±0.26;P<0.005) for indirectly calculated mean trabecular plate density, an index of the number and connectivity of structural elements, and as a necessary corollary, a significantly higher mean value for the mean thickness of structural elements. Plate density was more than one standard deviation below the age-adjusted mean value for normal postmenopausal white females in 19 (73%) of the fracture caes and in only 5 (21%) of the nonfracture cases (P<0.001). We conclude that the biomechanical competence of trabecular bone is dependent not only on the absolute amount of bone present but also on the trabecular microstructure.