The change in vertebral shape with increasing severity of spinal osteoporosis was studied in 139 women with atraumatic spinal fractures. The anterior height was below the normal range in 570 vertebrae, and the posterior height was below normal in 157 vertebrae. All values below the normal range were defined as fractures. Ninety-eight percent of posterior fractures were associated with anterior fractures. The fractures involved both the anterior and posterior vertebral borders in 58/356 (17%) of the fractured vertebrae in patients with 6 or less fractures, compared with 93/217 (45%) of the fractured vertebrae in patients with 7 or more fractures. The distribution of anterior fractures was biphasic with peak frequencies at T7-T8 and T11-T12. Posterior fractures had a more lumbar distribution, perhaps resulting from higher compressive forces on the posterior vertebral border of the lordotic lumbar spine than on the kyphotic thoracic spine. The mean anterior and posterior height of nonfractured vertebrae in osteoporotics was not significantly different from normal, suggesting that osteoporotics do not have smaller vertebrae than normal subjects.