NAGAI, N., TAKAYAMA, S., HIRATA, Y. and MARUYAMA, H. Classification and Heredity of Diabetes among Japanese Patients with Diabetes Found before 30-Year- Old. Tohoku J. exp. Med., 1984, 143(3), 289-294-Classification and family history of diabetes were examined in 551 Japanese patients whose diabetes were found before the age of thirty. These patients were treated at the Diabetes Center of Tokyo Women's Medical College from 1976 to 1981. Among those 551 diabetics, 198 patients were classified into insulin-dependent diabetes mellitus (IDDM) and 337 into non-insulin-dependent diabetes mellitus (NIDDM). IDDM was found mainly in the group of diabetics whose onset of diabetes was before the age of fifteen. The number of NIDDM was significantly increased after the age of twenty. Family history of diabetes was much less in the IDDM group than in the NIDDM group in the first degree relatives. To compare the family history of IDDM with that of NIDDM, 148 diabetics were selected from those 551 patients. The selected diabetics had the onset of the disease before 20 years of age, their present age was between 20 and 40, and the duration of diabetes of these patients was three years or longer. Six of the selected 75 diabetics with IDDM had parents with NIDDM and two of these 75 patients had siblings with IDDM. Thirty-nine of the selected 73 diabetics with NIDDM had positive family history of NIDDM alone. And the selected diabetics with earlier onset of IDDM showed higher risk for IDDM case in their siblings.