We estimated the medical expense of treating patients with diabetes mellitus under the health insurance system in Japan. The expense was summed up to 5.07 million yen for a patient who developed NIDDM at the age of 40 and died at the age of 75 without diabetic complications throughout his life. If he had diabetic retinopathy, neuropathy, hypertension and hyperlipidemia, and needed insulin injection, the cost would have increased 2.4 times to 12.32 million yen. It was also estimated at 25.22 million yen for a patient who developed IDDM at the age of 20 and died at the age of 70 without diabetic complications. If the patient had with diabetic retinopathy, neuropathy and hypertension and needed hemodialysis because of nephropathy for 20 years, the figure would have reached a whopping sum of 76.17 million yen. From the viewpoint of medical economy, more effort to prevent diabetic complications should be made.