The binding characteristics of insulin autoantibodies (IAA) were compared with those of antibodies to exogenous insulin (IBA) by analyzing the specific binding, binding capacity and affinity for insulin in 11 children (age range 1.5–13.0 years) with insulin‐dependent diabetes (IDDM) both at diagnosis and after 1 year of insulin treatment. Maximal specific insulin binding was 7.8 (1.5; SE) pmol^‐1 for IAA and 28.1 (6.7) pmol 1^‐1 for IBA (P<0.01), and the binding capacity of the high affinity class of IAA 0.01 (0.003) nmol l^‐1, as compared with 0.19 (008) nmol 1^‐1 for the corresponding IBA class (P< 0.01). With regard to the low‐affinity components the binding capacity was 0.11 (005) nmol l^‐1 for IAA and 1.50 (0.95) nmol l^‐1 for IBA (^P< 0.01). No differences in the affinity constants could be observed between IAA and IBA. There was no correlation between the insulin binding of IAA and quantitative levels of islet cell antibodies (ICA) at the clinical presentation or subsequent IBA values. The specific insulin binding of IBA correlated negatively with serum C‐peptide concentrations and positively with HbA₁ levels at 1 year. The present observations suggest that IAA developing before the diagnosis of IDDM are characterized by a reduced binding capacity as compared with antibodies to exogenous insulin, whereas they have a similar affinity for insulin. IAA seem to be quantitatively unrelated to ICA and postdiagnostic IBA levels. High IBA levels appear to be associated with reduced endogenous insulin secretion and poor metabolic control during the early clinical course of the disease.