We report two cases of obese young men with high serum insulin and acanthosis nigricans. Case 1 was a 16-year-old man (173 cm tall, 113 kg weight, BMI 37.7 kg/m²). A 75 g oral glucose tolerance test (75 g OGTT) showed impaired glucose tolerance and hyperinsulinemia (fasting IRI; 40μU/ml, highest IRI; 490μU/ml). Case 2 was a 21-year-old man (160.3cm tall, 91 kg weight, BMI 34.5kg/m²). A 75 9 OGTT showed normal glucose tolerance and hyperinsulinemia (fasting IRI; 81μU/ml, highest IRI; 250mU/ml). In both cases, hyperinsulinemia was improved after weight loss. Although missence mutations in the insulin receptor gene were not identified in eithercase, Trp64Arg heterozygous polymorphism in the β₃ adrenergic receptor gene and Pro12 homozygous polymorphism in the PPARγ gene were both identified in these two cases.