Vertebrates constantly remodel bone to maintain a constant bone mass. Bone remodeling comprises two phases: bone resorption by the osteoclasts followed by bone formation by the osteoblasts. Although the prevailing view about the control of bone remodeling is that it is an autocrine/paracrine phenomenon, the bone resorption arm of bone remodeling is under a tight endocrine control. To date little is known about the regulation of bone formation. We took the observations that gonadal failure favors bone loss and obesity protects from it as an indication that bone mass, body weight, and reproduction could be regulated by the same hormone(s). Leptin is one of these hormones. Leptin inhibits bone formation by the osteoblasts. This function is dominant, and leptin deficiency results in a high bone mass phenotype despite the hypogonadism characterizing these animals. Genetic biochemical and physiological studies demonstrate that leptin inhibits bone formation following its binding to its receptor in the hypothalamus. These results are the first evience that bone remodeling is a hypothalamic process; they imply necessarily that osteoporosis, the most frequent bone remodeling disease, is partly at least a hypothalamic disease. This finding also has therapeutic implications.