Obesity is a multifactorial disease that arises from complex interactions between genetic predisposition and environmental factors. It increases a risk of cardiovascular and metabolic diseases such as diabetes, hypertension, and hyperlipidemia. Recent molecular genetic studies have disclosed some monogenic forms of obesity in humans. Leptin directly exerts its anorexigenic effects on hypothalamic arcuate nucleus. alpha-melanocyte stimulating hormone (alpha-MSH) derived from proopiomelanocortin (POMC) and melanocortin-4 receptor (MC4-R) have been reported to be involved in the downstream of leptin actions. In this paper, we summarize the clinical characteristics and the mechanisms of obesity caused by genetic abnormalities in leptin receptor and melanocortin-4 receptor.