We compared the frequencies of antibodies to GAD (GADA) and IA-2 (IA-2A) and their titers at diagnosis in 48 Japanese children with slowly progressive form of type 1 diabetes (SPT1D) and 70 children with rapidly progressive form of type 1 diabetes (RPT1D). High prevalences of both GADA and IA-2A were found at diagnosis in both the patients with SPT1D (70.8% and 75.0%), and those with RPT1D (71.4% and 71.9%). Most patients, regardless of the form of type 1 diabetes, were positive for both antibodies, though 6 of the 9 patients less than 5 years of age were negative for both antibodies. GADA titers below 50 U/ml were significantly more frequent in the patients with SPT1D (79.4% vs. 38.0%, p=0.0002), and titers above 100 U/ml significantly more frequent in those with RPT1D (38.0% vs. 11.8%, p=0.0081). No significant association was noted between the titers of IA-2A and the clinical form of type 1 diabetes. These results suggest that low GADA titers may reflect mild autoimmune destruction of beta-cells with slow disease progression. Titers of IA-2A do not appear to reflect the degree of autoimmune damage of the beta-cells.