This case report describes an unusual presentation of nausea, vomiting and diarrhea, which was misdiagnosed as acute gastroenteritis in a 6-year-old girl. The patient later returned to the emergency department (ED) with severe dehydration from idiopathic central diabetes insipidus (DI). At her first visit, this previously healthy patient presented with mild dehydration, signs of acute gastroenteritis and normal urine output. Her brother had experienced similar symptoms a few days earlier. She tolerated an oral fluid challenge and was discharged from the ED with stable vital signs. Two days later, the patient returned with severe dehydration, resolving diarrhea and persistent vomiting. She was admitted to the pediatric intensive care unit. Magnetic resonance imaging illustrated an absent posterior pituitary enhancing signal, which demonstrated a loss of function in that region. There were no other abnormalities. The patient subsequently received desmopressin with improving clinical status and was discharged on the eighth hospital day. DI is a rare disease, but can be fatal if left undiagnosed. It should, therefore, be considered in the differential diagnosis of a dehydrated patient with an unexpectedly low urine specific gravity.