Autoantibodies to glutamic acid decarboxylase (GAD), autoantibodies to 64 kDa islet cell protein and islet cell antibodies (ICA) were measured in 79 Japanese patients with insulin-dependent diabetes mellitus (IDDM). The overall prevalences of GAD antibodies, 64K antibodies, and ICA in these patients were 69.6% (55/79), 48.1% (38/79), and 46.8% (37/79), respectively. However, in a subset of these patients with recent onset IDDM (< 1 year) the prevalences of GAD antibodies, 64K antibodies, and ICA were 78.8% (26/33), 66.7% (22/33), and 78.8% (26/33), respectively. Furthermore, the prevalences of GAD antibodies, 64K antibodies, and ICA were significantly decreased in patients with long standing diabetes at 60.9% (28/46), 34.8% (16/46), and 23.9% (11/46), respectively. However, when these patients were divided into two groups by the presence or absence of organ-specific autoimmune disease (OSAD), the mean levels of GAD antibodies and ICA in the patients who gave a positive result were significantly higher in patients with OSAD (397 units and 98 JDF units, respectively) than in patients without OSAD (74 units and 39 JDF units, respectively). These results demonstrate that it is important to evaluate the prevalences and levels of islet-specific autoantibodies when considering disease duration and co-existence of autoimmune disease in patients with IDDM.