Abstract Background: Leptin is secreted by white adipocytes in response to fat storage and binds to leptin receptors (LEPR) expressed all over the body particularly in hypothalamic neurons. This hormone regulates several physiologic functions including energy expenditure and appetite. Clinical Case: We describe the clinical and hormonal findings in 3 adult brothers with body mass index (BMI) of 36.7, 50.7, and 46.1 kg/m2, respectively. There is no history of consanguinity in the family and none of the patients exhibited dysmorphic features. A rapid weight gain was noticed during their first few months of life, associated with permanent hyperphagia. At 2 years old, their BMI was already above 25 kg/m2 (> +3SD), and at 10 years old, it was over 40 kg/m2 (> +3SD). The linear growth was within the expected target heights for their parents. They did not seem to present cognitive impairment. The pubertal development began at 16 to 18 years old, and since then they maintained levels and FSH and LH above the upper limit of normal (15.6±3.7mUI/mL and 12.3±2.2mUI/mL, reference range 1.5–12.4 and 1.7–8.6, respectively), but with normal sexual steroids (estradiol 36.4±16.1pg/mL and total testosterone 445±401ng/dL, reference range 11–44 and 249–836, respectively). The thyroid function was normal and none of the patients suffered from dyslipidemia or diabetes, despite high serum insulin levels 26.4±15.8 mU/L (normal 5–10). Genetic sequencing identified a homozygous mutation of the leptin receptor gene in the 3 brothers: c.2357T> C, p. (Leu786Pro). Their parents were heterozygous for the mutation as well as the patients’ daughters. Homozygous carriers of the mutation presented a significantly higher BMI than their heterozygous family members, 44.5±7.1 Kg/m2 vs 32.2±1.7 Kg/m2 (p=0.023), a significantly increased leptin levels, 80±36.4 ng/ml vs 26.3±9.3 ng/ml (p=0.028), and significantly higher weight, 134.6±16.9 vs 89.2±15,2 kg (p=0.021), respectively. Women had higher BMI than men (42.0 ±12.2 Kg/m2 vs 37.9±7.5 Kg/m2, p=0.496) and also higher percentage of fat (46.4±3.1% vs 34.9±6.8%, p=0.108). Serum levels of leptin in homozygous patients were not significantly higher than those measured in 10 patients with adult-onset morbid obesity, 80± 36.4 ng/ml vs 53.8±24.1 ng/ml, respectively (p=0.149). Therefore, serum leptin is not a useful discriminative maker of LEPR gene mutations. Conclusion: Patients with severe early-onset obesity should have a genetic diagnosis workup. Firstly, because clinical trials with MC4R-agonists have raised expectations regarding the treatment of patients with mutations of the LEPR gene. Secondly, and in contradiction to other reports in the literature, our patients were fertile. Therefore, identification of the mutation allows genetic counseling of these patients and their families, including the possibility of Pre-Natal Diagnosis or Pre-Implantation Genetic Diagnosis.