Calcitonin levels were studied in 18 patients with chronic renal insufficiency periodically hemodialysed, comparatively with 9 only drug-treated uremic patients. Calcitonin was radioimmunologically assayed before and after hemodialysis and repeated 5 months later in some of the patients. Serum calcitonin was increased in most of the studied patients, especially in the dialysed ones and with a creatinine clearance below 10 ml/min. Calcitonin concentration was all the more increased as the dialytic treatment lasted longer. Considering the increasing role of calcitonin as osteoprotector, these findings might be of interest in the treatment of renal osteopathy.