A 60-year-old man was admitted to the Kyushu University Hospital because of poor glycemic control of diabetes mellitus. Although he had been treated with glicrazide and nifedipine for his diabetes and hypertension, the controls of the diseases were unsatisfactory. Plasma triglyceride level was 186 mg/dl. Furthermore, extreme insulin resistance was found by measuring glucose infusion rate with an euglycemic hyperinsulinemic clamp method. These findings were compatible to those seen in syndrome X. After admission, diet therapy of 1,800 Cal was started and his metabolic disorders such as hyperglycemia, hyperlipidemia, and hypertension were all improved. Moreover, euglycemic hyperinsulinemic clamp study also revealed a decreased insulin resistance after diet therapy. Our experience from the case suggested that insulin resistance may closely related with the metabolic disorders of the disease "syndrome X".