The available methods to quantitate vertebral deformity in osteoporotics are not satisfactory in comparing follow-up measurements in patients. This paper describes a newly developed 'spine deformity index' (SDI) which allows the quantitation of the extent of vertebral fractures. It is based on the observation that, in 110 normal persons, the heights of all vertebral bodies were related to each other in a predictable and constant manner. This relation was independent of the body height of the individual and was preserved despite growth acceleration during the last century. Since in all but one of our osteoporotic patients the 4th thoracic vertebra was unfractured we were able to compare the actual size of their fractured vertebrae to the calculated presumable original heights. The differences between presumable original and actual heights gave a measure of the extent of vertebral compression and allowed to define an index representing the sum of all spinal fractures in osteoporotics. The method was applied retrospectively to X-rays of 39 patients with idiopathic osteoporosis. Thirty-two of them were treated orally with 80 mg sodium fluoride, 1,000 mg calcium and 3000 IE vitamin D daily. Treatment resulted in a reduction of the progression of vertebral deformity. Seven inadequately treated patients had more pronounced progression of vertebral deformity.