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Carboxyl-terminal parathyroid hormone (C-PTH) fragments constitute 80% of circulating PTH. Since the first 34 amino acids of the PTH structure are sufficient to explain PTH classical biological effects on the type I PTH/PTHrP receptor and since C-PTH fragments do not bind to this receptor, they have long been considered inactive. Recent data suggest the existence of a C-PTH receptor through which C-PTH fragments exert biological effects opposite to those of human PTH(1–84) on the type I PTH/PTHrP receptor. This is why a lot of attention has been paid to these fragments recently.In vivo, synthetic C-PTH fragments are able to decrease calcium concentration, to antagonize the calcemic response to human PTH(1–34) and human PTH(1–84) and to decrease the high bone turnover rate induced by human PTH(1–84). In vitro, they inhibit bone resorption, promote osteocyte apoptosis and exert a variety of effects on bone and cartilaginous cells. These effects are opposite to those of human PTH(1–84) on the PTH/PTHrP type I receptor. This suggests that the molecular forms of circulating PTH may control bone participation in calcium homeostasis via two different receptors. Clinically, the accumulation of C-PTH fragments in renal failure patients may cause PTH resistance and may be associated with adynamic bone disease. Rare parathyroid tumors, without a set point error, overproduce C-PTH fragments. The implication of C-PTH fragments in osteoporosis is still to be explored.C-PTH fragments represent a new field of investigation in PTH biology. More studies are necessary to disclose their real importance in calcium and bone homeostasis in health and disease.