Synopsis: Insulinoma is the most common cause of hyperinsulinemic hypoglycemia. We report a 23 year-old male with a five-year history of seizures, documented hyperinsulinemic hypoglycemia with negative imaging studies. Clinical Presentation: We report a 23- year old male nursing student with 5-year history of seizures. Five years prior to admission, patient was found unconscious and diaphoretic at dawn. Capillary blood glucose (CBG) was noted to be 28 mg/dl with reversal of symptoms after intravenous glucose administration. Subsequently, hypoglycemic episodes with glucose levels ranging between 20-30 mg/dl, which were unrelated to food intake. The episodes occurred thrice weekly (midnight - 6a.m.) predominantly after hospital duties, Which abated with carbonated drinks. He had three admissions for unconsciousness and seizures. Pancreatic insulinoma was considered. On the 10th hour of 72-hour fast, he was diaphoretic with blood glucose (28-35 mg/dl), and inappropriately elevated insulin 66.1 (NV < 7.1uU/ml) and C-peptide 6.68 (NV 1.1-5ng/ml). Abdominal ultrasound and magnetic resonance imaging (MRI) revealed normal pancreas. Physical Findings: Examined an obese patient with BMI 31.6 kg/m2, and waist-hip-ratio 0.97 with normal vital signs, unremarkable physical and neurologic findings. Diagnostics: Intra-arterial calcium gluconate stimulation test with hepatic venous sampling was performed stimulating the hepatic, gastroduodenal, superior mesenteric, proximal and distal splenic arteries with calcium gluconate (0.025mEq/kgBW). Baseline insulin levels in all arteries were 8.9 - 10.8 fold elevated (313.2 - 375.4 uIU/mL). A 1.4- fold increase at 60 seconds was noted in the superior mesenteric artery. Treatment: Patient underwent exploratory laparotomy where a 2 cm mass posterior to the pancreatic neck was palpated. Intraoperative ultrasonography revealed a 1.2x1.9cm sonolucent mass in the transverse and longitudinal planes. He underwent near-total pancreatectomy and splenectomy. Intraoperatively, CBGs ranged between 120-150 mg/dL with a linear rise to 200 mg/dL 60 minutes after resection of the pancreatic tumor. Serum insulin decreased to 51.1uIU/mL post-operatively. Histopathology revealed pancreatic islet cell tumor (insulinoma) confirmed by synaptophysin and chromogranin A staining. Outcome: One month post-surgery, he weighed 64kg and required 18 units of basal insulin to maintain euglycemia. He has had no recurrence of seizure since after surgery.