© 2017 2017 Walter de Gruyter GmbH, Berlin/Boston.Monogenic obesity results from single gene mutations. Extreme obesity starting at an early age, especially in infancy, which is associated with endocrinopathy and metabolic disturbances is key to the diagnosis of monogenic obesity. A 6-month-old boy was admitted to our clinic with severe obesity and food craving. He was born with a birth weight of 3400 g to first-cousin parents. He started to gain weight at an abnormal rate at the age of 2 months. He had hyperinsulinemia, dyslipidemia and grade 2 hepatosteatosis. He had a 7-year-old, healthy brother with a normal body weight. Because of severe early-onset obesity and abnormal food addiction, his leptin level was measured and found to be 0.55 ng/mL (normal range for his age and sex is 0.7-21 ng/mL). A LEP gene mutation was screened for and a gross leptin gene deletion was detected. To date, no report on a gross deletion of the LEP gene has been published in the literature. To the best of our knowledge, a gross deletion of the LEP gene has not been reported so far in the literature. Here we report a unique case with congenital leptin deficiency. Thus, clinicians should search for monogenic obesity in patients with early-onset severe obesity and endocrinopathy. Measuring the leptin level could aid clinicians to identify children with monogenic obesity.