Background: Type I diabetes (DM1) is caused by an autoimmune process that destroys beta cells of pancreas. Not all carriers of susceptible HLA genes and positive for autoantibodies develop the disease. Environmental factors play a role in triggering the autoimmune process. Aim.. To analyze an exceptional case of DM1 in a Mapuche family in the context of genetic, immunological and environmental factors. Subjects and methods: A study of a family with an affected female child was carried out in a Mapuche community in Southern Chile (VIII region). This is an unique and sporadic DM1 case with Mapuche heritage. Nutritional and viral infections data were collected by interview and clinical records. A genetic analysis by PCR was done to detect class I and II HLA genes by reverse dot blot. Results: The proband, her mother and sister had positive islet cell antibodies (ICA.). Her father and brother were negative. All the family was positive for anti glutamic decarboxylase antibodies (GAD65). All subjects bad HLA-DRB1 0407/0407 and HLA-DQB1 0302/0302 alleles. The index case and her father were homozygotes for the HLA-A1:A*68012/A*68012 allele. Mean breast feeding lapse was 18 months in all children. No evidences for viral infections such as rubella, mumps or measles were found in this family. Conclusions: There was an altered profile of autoantibodies in the family of the index case. All genotypes were comparable with the European population where the diabetogenic combination DR4/DQB1*0302 is the most prevalent. No environmental factors could be incriminated as triggers of the disease (Rev MM Chile 2004; 132: 47-50).