Context: Dietary factors may trigger or exacerbate the autoimmune disease process. Objective: To examine dietary glycemic index and glycemic load for association with islet autoimmunity (IA) development, and progression from IA to type 1 diabetes. Design: The Diabetes Autoimmunity Study in the Young (DAISY) follows children at increased genetic type 1 diabetes risk. Diet is collected prospectively via parent-reported food frequency questionnaire (FFQ). Setting: Observational study of children in the Denver area. Patients: 1,776 DAISY children age <11.5. Interventions: None. Main Outcome Measures: IA, defined as presence of autoantibodies to insulin, GAD or IA2 at two consecutive visits, or presence of autoantibodies at one visit and diabetic on the next consecutive visit. Type 1 diabetes was diagnosed by a physician. Eighty-nine subjects developed IA, and 17 subsequently developed type 1 diabetes during follow-up. Hypothesis formulated after data collection. Results: Glycemic index and glycemic load were not associated with IA development. More rapid progression to type 1 diabetes in children with IA was associated with higher dietary glycemic index (Hazard Ratio (HR): 2.20, 95% Confidence Interval (CI): 1.17–4.15), and marginally associated with glycemic load (HR: 1.59, 95% CI: 0.96–2.64) at first IA positive visit. Conclusions: Higher dietary glycemic index and glycemic load are not associated with IA development, but higher glycemic index is associated with more rapid progression to type 1 diabetes in children with IA, perhaps due to increased demand on the -cell to release insulin. Further study is needed to confirm this finding and identify the underlying biologic mechanism.