As the incidence rate of msuli-dependent diabetes mellitus (IDDM) is very low in Koreans, there have been little data on the peculiarity ot IDDM in Korea. To elucidate the characteristics of IDDM in Korea, we performed HLA serotyping for HLA-A, B. -C and -DR oci, examined islet cell cytoplasmic antibody (ICA) and islet cell surface antibody (ICSA) and analyzed clinical features in 61 patients with young adult-onset. IDDM. Twenty hundred and eighty unrelated volunteers were included as healthy controls. The frequency of HLA-DR4 in the patients was higher than that in the controls (57,4% vs 36.4% relative risk 2, 3, corrected p<0.05) and the frequency of HLA-DR2 was lower in the patients as compared withr the controls (13.1% vs 32.1%, relative risk 0.3, corrected p<0.05). There were no significant differences between the DR4 -positive patients the DR4-negative patients in age at onset,sex, ratio,history of diabetic ketoacidosis (DKA), family history of diabetes,current body mass index (BMI), levels of basal serum C-peptide and 24-hour urinary C-peptide, and prevalences of ICA and ICSA, Eighteen of 61 patients (29.5%) were positive for ICA and twentyone (34.4%)wrere positive for ICSA. There was a negative relation between the frequencies of islet cell autoantibodies and the duration of diabetes. Only 28 of All patients (45.9%) presented with IDDM at the time of diagnosis of diabetes. In contrast, the other patients (54.1%) initially preseeged with NIDDM and progressed through the non insulin dependent stage of 3.0 years, rangieg from 0.7 to 10.0 years, en route to IDDM. However, we did found no differencee between the two groups in age at onset, sex ratio, history of DKA, famiIy history of diabetes, corrent BMI, current insulin dose, levels of batal serurn C-peptide and 24-hour urinary C-peptide, ftequencies of HLA-BR4 and HLA DR2, and, frequencies of cell autoantibodies. We conclude that IDDM that begins in young aduIt- hood is characterized by positive association with HLA-DR4, negative association with HLA-DR2 presence of the islet specific autoantibodies, and the heterogeneity in the mode of onset and the earlier clinical course.