Background: Islet cell-specific autoantibodies such as islet cell antibody (ICA), antiinsulin (IAA), anti-glutamic acid decarboxylase (GAD) and anti-tyrosine phosphatase (IA2) can be present in patients with type I diabetes. Breast feeding duration and the early exposure to milk substitutes are environmental factors associated to etiology of type 1 diabetes. Aim To study the frequency of the anti-GAD, anti-IA-2 e ICA antibodies in Chilean type 1 diabetic patients and determine the possible modulator effect of the breast feeding. Patients and methods: One hundred thirty four type I diabetic patients, aged one to 15 years old, were studied at the moment of their diagnosis. Patients were classified according to the duration of exclusive breast feeding. IA-2 and GAD were determined by radio immuno assay and ICA by means of indirect immunofluorescence. Results: Subjects with three months or less and those with more than three months of breast feeding were positive for ICA in 78.8 and 90.6% of cases respectively, for GAD in 75 and 54.6% of cases respectively (p=0.024) and for IA-2 in 73 and 43.8% of cases respectively (p=0.001). All three antibodies were positive in 53.9 and 21.8% of children with less or more than three months of breast feeding (p=0.001). Conclusion: Both IA-2 and GAD antibodies are less frequently positive in type 1 diabetic patients who have been breast fed for more than three months. These findings suggest a possible attenuating role of exclusive breast feeding on pancreatic aggression events in patients with type 1 diabetes (Rev Méd Chile, 2001; 129: 611-9)