A 4-year-old chestnut-fronted macaw (Ara severa) was examined for polyuria, polydipsia, and weight loss. Abnormal clinicopathologic findings, including leukocytosis, anemia, β-globulinemia, hyperglycemia, glucosuria, and ketonuria, were consistent with diabetes mellitus. Increasing doses of neutral protamine Hagedorn (NPH) insulin had no effect on the hyperglycemia, and the macaw died 5 days after presentation. Histologic examination of the pancreas at necropsy showed marked islet cell vacuolar degeneration, multifocal lymphoplasmacytic inflammation, and exocrine pancreatic hyperplasia. Immunohistochemistry performed on the pancreas revealed nearly complete absence of insulin immunopositive cells. The pancreatic lesions were consistent with type 1 diabetes mellitus.