To describe our 10-year experience in treating leptin-deficient humans. Three adults and one boy presented with childhood-onset morbid obesity, hypogonadism and family history of obesity and early death. Serum leptin was inappropriately low. A recessive C105T leptin gene mutation was identified. Metabolic and endocrine assessments were conducted, before and while on and off leptin. The adults' body mass index decreased from 51.2 ± 2.5 to 29.5 ± 2.8 kg/m(2). Serum lipids normalized, insulin resistance decreased, and one of the initially diabetic females became normoglycemic. Hypogonadotropic hypogonadism was reversed, and other changes were observed in the adrenal, sympathetic, somatotropic and thyroid functions. Leptin replacement therapy reverses endocrine and metabolic alterations associated with leptin deficiency. Some of these results may be extrapolated to other diseases.