Insulin autoimmune syndrome (IAS) or Hirata's disease is a rare cause of hyperinsulinemic hypoglycemia. We report the case of a child with a mild, atypical presentation of IAS. A previously healthy girl, aged 7 years old, developed non-ketotic fasting hypoglycemia during treatment for pneumonia. Laboratory evaluation during hypoglycemia showed the following results: serum glucose, 32 mg/dL (1.8 mmol/L); insulin, 5.6 μIU/mL (38.9 pmol/L); C-peptide, 1.4 ng/mL (0.47 nmol/L); anti-insulin antibody, 6.2% (normal, <2.4%); absence of ketonuria; and positive glucagon stimulation test result. Search for mutation in genes ABCC8, KCNJ11, GLUD1 and MEN1 was negative. Human leukocyte antigen (HLA) typing was HLA-DRB1*1104. Computed tomography scan of the abdomen showed a normal result. The patient evolved with spontaneous resolution of the hypoglycemia, within 30 days, with normalization of serum anti-insulin titers. The serum levels of insulin and anti-insulin antibodies in the patient of this report were not extremely high as previously reported. This novel, mild, or forme fruste presentation of IAS expands the previously reported spectrum of this disease.