Objective We aimed to characterize insulin responses to intravenous glucose during the preclinical period of type 1 diabetes starting from the emergence of islet autoimmunity. Design and methods A large population-based cohort of children with HLA-conferred susceptibility to type 1 diabetes was observed from birth. During regular follow-up visits islet autoantibodies were analysed. We compared markers of glucose metabolism in sequential intravenous glucose tolerance tests between 210 children who were positive for multiple (≥2) islet autoantibodies and progressed to type 1 diabetes (progressors) and 192 children testing positive for classical islet-cell antibodies (ICA) only and remained healthy (non-progressors). Results In the progressors, the first phase insulin response (FPIR) was decreased as early as 4 to 6 years before the diagnosis when compared to the non-progressors (P=0.001). The difference in FPIR between the progressors and non-progressors was significant (P<0.001) in all age groups, increasing with age [at 2 years: difference 50% (95% CI 28-75%); at 10 years: difference 172% (95 CI 128-224%)]. The area under the 10 minute insulin curve showed a similar difference between the groups (P<0.001) [at 2 years: difference 36 % (95% CI 17-58 %); at 10 years: difference 186% (95% CI 143%-237%)]. Insulin sensitivity did not differ between the groups. Conclusions FPIR is decreased several years before the diagnosis of type 1 diabetes, implying an intrinsic defect in β-cell mass and/or function.