We have previously demonstrated that genetically based leptin deficiency due to a missense leptin gene mutation in a highly consanguineous extended Turkish pedigree is associated with morbid obesity and hypogonadism. We have now performed detailed assessments of endocrine, sympathetic, and immune function. We have also identified a new adult female homozygous patient in this extended family who is severely obese and amenorrheic. In this family all wild-type and heterozgous individuals have normal body weight. Seven obese members of this family, whom we presume to have been leptin deficient, died during childhood. There are several findings that indicate potentially novel targets for leptin action in humans. Four homozygous patients (1 adult male, 2 adult females, and 1 child) have sympathetic system dysfunction, whereas all heterozygous subjects have normal sympathetic system function. Despite sympathetic system dysfunction and postural hypotension, 1 of 3 homozygous adult patients has impaired renin-aldosterone function. The patients also exhibit alterations in GH and PTH-calcium function, and 1 of them has decreased bone mineral density. Despite their obesity, these patients do not have risk factors for cardiovascular disease, such as hypertension, impairments in lipid metabolism, or hyperleptinemia [corrected]. These data support the hypothesis that the obese may have central, but not peripheral, resistance to the effects of leptin and that hyperleptinemia [corrected] may mediate the cardiovascular morbidity of the obese who are not leptin deficient. Furthermore, these data indicate that there may be several new targets for leptin action in human physiology. Such new targets may lead to novel pharmacological strategies for the use of leptin agonists and antagonists in the treatment of human disease. All 19 normal weight individuals in this family are alive, whereas 7 of 11 obese individuals died in childhood after infections. The odds ratio for mortality in the context of this obesity phenotype is 25.4, indicating that this mutation severely impairs key biological functions during childhood, negatively impacting on survival. We found that only the obese child in this family had thyroid function abnormalities. The oldest homozygous female patient started to menstruate, albeit with a luteal phase defect, 7 months ago, after a delay of over 20 yr, whereas the younger adult subjects are still hypogonadic. Thus, we conclude that due to their long life span, humans who survive the negative effects of leptin deficiency during childhood can, in contrast to ob/ob mice, over decades compensate some of the effects of leptin deficiency on immunity and endocrine function through mechanisms that remain to be elucidated.