Calcitonin is a peptide hormone secreted by the C-cells of the thyroid gland. A major physiological function of the hormone appears to be the protection of the skeleton against resorption in humans. It thus opposes the resorptive actions of parathyroid hormone and 1,25 dihydroxyvitamin D. This action is utilised pharmacologically in order to treat diseases where increased bone resorption is a major component. The efficacy of calcitonin in the treatment of Paget's disease of bone is well established, especially as it is currently the most effective agent in the treatment of the osteolytic form of the disease. In other bone diseases where resorption of bone is a component it is likely to be worthy of a trial of therapy. There are also sound theoretical reasons why calcitonin may be of benefit in the treatment of certain osteoporoses, especially in combination with other agents. Most recent studies would seem to support his view. A disadvantage of calcitonin therapy is that the hormone has to be administered parenterally, although future developments may obviate this. It is, however, a form of retreatment which is free of any long term serious side effects, and calcitonin now has a definite place in the management of specific bone and calcium disorders.