A 15-year-old female patient with known type 1 diabetes mellitus was referred because of abdominal pain. On admission, she was alert but dehydrated with marked Kussmaul breathing. Blood glucose was 414 mg/dL (23 mmol/L). Blood gas analysis revealed severe metabolic acidosis (pH: 6.99) with an elevated anion gap (29.8 mmol/L) and an increased base excess (-25.2 mmol/L). At the sixth hour of treatment with intravenous fluids and insulin, the patient became delirious. The delirium persisted despite the normalization of the acidosis and became difficult to manage. Brain imaging studies revealed neither brain edema nor other intracranial pathology. No evidence of intoxication could be found. The patient gradually regained consciousness and was diagnosed as a case of severe diabetic ketoacidosis (DKA) associated with infection. We were unable to find a similar case in the pediatric literature and thought that reporting this unusual case would be a contribution to the literature on DKA in children.