Bone mineral density of the lumbar vertebrae (L2-4BMD) in involutional osteoporosis was measured by dual photon absorptiometry to investigate its relationship with the incidence of osteoporotic vertebral fractures of the thoracic and lumbar spine. In addition to monitor L2-4BMD, the author also examined the progress of vertebral fractures during drug treatments by means of radiographs. L2-4BMD at the antero-posterior projection was measured in 287 female subjects with involutional osteoporosis ranging from 45 to 91 years of age. Fracture of the vertebrae was determined from lateral radiographs. Low BMD is the most important risk factor in vertebrae fractures. There was a significant difference in L2-4BMD with and without fractures of the vertebrae. L2-4BMD for threshold was 0.633 g/cm2 for 90th percentile with vertebral fractures, and that for breakpoint was 0.532 g/cm2 for 65th percentile with vertebral fractures. The incidence of vertebral fractures with BMD above the threshold was 17.5%, however that between the threshold and the breakpoint was 30.6%. Furthermore that below the breakpoint was 48.9%. Although the incidence of vertebral fractures was affected to some extent by age, height, and weight, the number of vertebral fractures increased with the decrease of L2-4BMD. The progress of vertebral fractures under drug treatments with 1,25-dihydroxyvitamin D3, ipriflavone, and calcitonin was affected by pre-existing vertebral fractures and L2-4BMD at the first visit. Estimation of L2-4BMD and the existence of vertebral fractures is important in predicting new vertebral fractures and the further collapse of pre-existing vertebral fractures.