Background: It has been shown that prolonged daily peroral magnesium (Mg) administration, as tabs of Mg(OH)(2), used as the only treatment in postmenopausal osteoporosis, causes a significant increase in BMD. Objective: In order to obtain definitive evidence of causality of magnesium deficiency in the etiology of osteoporosis, we spent 1 year examining rats given a daily Mg-deficient diet (200 ppm) and compared them with rats given a Mg-adequate diet (2000 ppm). Methods: Sixteen female Sprague-Dawley rats, mean weight 110 (S.D. 23) g, were divided into two groups and randomly assigned to a semisynthetic diet that differed only in Mg content. Urine samples were collected every 3 months and blood was collected at the end of the trial. After the animals were sacrificed, the L3-L5 vertebrae and the femoral regions were examined for bone density (BMD) using dual energy X-ray absorptiometry. The femoral bones were examined for bone fragility and the tibiae by histomorphometry, and the mineral content of the bones was estimated. Results: The mean BMD of L3-L5 vertebral bone was significantly higher in group A (adequately nourished) than in the Mg-deficient group B (p=0.035, one-tail); in addition, the BMD of the femural region was significantly higher in group A (p=0.045, one-tail). The bending stiffness of the femur was slightly higher in group A than in group B; however, after correction to diminish the influence of the difference in bone dimensions between the two groups, femur rigidity (i.e., the loss of elasticity) in group B became significantly higher than that in group A (p=0.024). The force needed to break the bone was significantly higher in group A than in group B (p=0.024) and it remained higher after correction, although no longer significantly. In group B, the diminution of the trabecular bone volume, in relation to tissue volume (BV/TV), and the increase in the degree of trabecular interconnection (TBPf) clearly showed the presence of osteoporosis. On microscopy, focal osteoporosis of the metaphyseal spongious bone was observed in the bone rendered Mg-deficient. Conclusions: Because osteoporosis is characterized by lowering of BMD, increased bone fragility, and altered bone architecture, our study showed that maintaining rats for 1 year on a Mg-deficient diet gives rise to the appearance of osteoporosis.