Insulinomas are functional neuroendocrine pancreatic tumors rarely encountered in pediatric pathology. Insulinomas are usually solitary and sporadic, but may occur in association with multiple endocrine neoplasia type 1. Whipple's triad-hypoglycemia, simultaneous compatible adrenergic and/or neurological signs, and relief of symptoms upon the administration of glucose-remains the fundamental diagnostic tool. We report a case of insulinoma in an 11-year-old boy with malnutrition and mild psychic retardation. History revealed neuroglycopenic symptoms associated with hypoglycemia that returned to normal values after glucose intravenous infusion; before admission in our unit, the levels of circulating insulin, as well as the abdominal ultrasound and abdominal computed tomography scan, were reported within normal range. During hospitalization in our service, the glycemic curves showed recurring low values associated with low glycated hemoglobin, positive fasting test, and elevated C-peptide. The pancreatic ultrasound was inconclusive, but the magnetic resonance imaging revealed a high signal focal area with a diameter of 1 cm, located in the tail of pancreas. Conventional enucleation of the lesion prompted a spectacular normalization of glucose metabolism and the alleviation of the main clinical symptoms. The child had a favorable evolution in the clinical follow-up, presenting with weight gain and progressive remission to complete disappearance of most symptoms-except for the mental impairments. Although in our case Whipple's triad was apparent from the beginning, the diagnosis was delayed due to the failure of conventional imaging methods in locating the tumor. Weight loss and mental impairment contributed to the diagnosis pitfalls. Pediatricians should be aware of confusing and nonspecific symptoms, especially when children with insulinoma present mental or neurological retardation. Despite the existence of medical regimens, surgery remains the gold standard for the therapeutic approach to this condition.