Changes in the calciotropic hormones with age contribute significantly to the pathogenesis of osteoporosis. In both postmenopausal (Type I) and senile osteoporosis (Type II) it is common to find reduced levels of serum 1,25-dihydroxyvitamin D and malabsorption of calcium. In Type I patients a reduced level of serum parathyroid hormone causes a real decrease in serum 1,25-dihydroxyvitamin D production and malabsorption of calcium, whereas in Type II patients the decline in 1 alpha-hydroxylase activity in the kidney causes a decline in serum 1,25-dihydroxyvitamin D which leads to malabsorption of calcium and secondary hyperparathyroidism. In the final analysis both pathways lead to bone loss. In some Type II patients there may be a decline also in the function or number of the vitamin D-binding receptors in the gut. Treatment of patients with vitamin D analogues, however, normalizes calcium absorption and improves calcium balance. The improvement in calcium balance reduces bone resorption and prevents further bone loss; in addition recent studies have shown that therapy with vitamin D analogues leads to a reduction in fracture incidence.