Bone mineral density (BMD) measurement of the lumbar spine by dual-photon absorptiometry (DPA) using a radioactive source and more recently an x-ray tube (DEXA) is a useful noninvasive technique to assess bone loss in vertebral osteoporosis. Because an anteroposterior (AP) projection is used, DPA measures not only the mainly trabecular bone of the vertebral body but also the cortical bone of the posterior processes, which does not contribute to the development of crushed fractures. Using a DPA apparatus equipped with a 153 Gd source coupled with an 18-detector system that provides better collection efficiency for the narrow beam of 153 Gd radiation, we have developed a technique measuring the BMD of the vertebral body of L2, L3, and L4 on a lateral projection based on previous studies using radiographic photodensitometry for lateral spine. The precision of the method-1.7% in vivo and 0.88% in vitro-is similar to that obtained with the AP projection (respectively, 1.7 and 1.0%). Lateral BMD decreased with age in 143 normal women and was best accounted for by a cubic curve (r = -0.51, p less than 0.001). The age-related bone loss between 30 and 80 years of age was much higher with the lateral (-44%) than with the AP (-22%) projection, a pattern consistent with the greater trabecular than cortical age-related bone loss. When osteoporotics (OP) were compared to age-matched normal women, the decreased in BMD was larger with the lateral (-30%) than with the AP projection (-23%), but in terms of Z scores, no difference could be found between both projections.(ABSTRACT TRUNCATED AT 250 WORDS)