We report an interesting and unique case of an overweight adolescent with a novel mutation of the maturity-onset diabetes of the young (MODY)3 gene [hepatocyte nuclear factor-1 alpha (HNF-1alpha)] and positive islet cell autoantibodies. The patient is a 17-yr-old Caucasian female, who was diagnosed with type 2 diabetes mellitus, treated with metformin and glipizide, with poor control for 18 months prior to being referred to the Endocrinology clinic. Family history was strongly positive for type 2 diabetes (father, paternal aunts, uncles, and grandmother). All were diagnosed at age 40-50 and treated with oral hypoglycemic agents. The patient's body mass index was 36.4 kg/m(2). She had no acanthosis nigricans. Initial hemoglobin A1c was 11.9%, with fasting glucose of 234 mg/dL and fasting insulin 10.7 microU/mL. She was started on insulin therapy (0.6 units/kg/d), resulting in good glycemic control. Oral hypoglycemic agents were discontinued. Immunologic studies showed positive islet cell (29 U/mL, normal <1.0) and glutamic acid decarboxylase-65 (0.9 U/mL, normal <0.5) antibodies. Sequencing for HNF-1alpha gene revealed a nucleotide A to G substitution (ACC to GCC), resulting in a missense mutation, T196A. To our knowledge, T196A has not been previously reported. The coexistence of type 1 diabetes autoimmunity and a mutation in the gene responsible for MODY3 in this overweight patient is intriguing and might explain the early onset of progressive insulinopenia compared with the later age of diabetes onset of the earlier generation in the family.