Abstract Disclosure: Y. Cho: None. K. Jo: None. J. Han: None. S. Moon: None. E. Kim: None. Background: Nesidioblastosis is a rare condition that should be considered in cases of pancreatogenic hyperinsulinemic hypoglycemia without evidence of insulinoma on imaging. We present the first case of nesidioblastosis concomitant with anatomically confirmed metastatic gastric cancer of the pancreas. Case Report A 57-year-old male presented with recurrent episodes of loss of consciousness and hypoglycemia, with a glucose level of 26 mg/dl upon arrival at the emergency room. Despite a history of advanced gastric cancer and previous chemotherapy, the patient had not been diagnosed with diabetes and was not taking medications associated with hypoglycemia. Four months prior, he underwent total gastrectomy. The fasting plasma glucose levels was 38 mg/dl, accompanied by C-peptide levels of 6.10 ng/mL, insulin levels exceeding 1000 uIU/ml, and proinsulin levels at 8.8 pmol/L. The insulin antibody level was more than 100%, and the highest cortisol level reached 15.60 ug/dL during the rapid ACTH stimulation test. The patient's postprandial 2-hour glucose level reached 233 mg/dl during the 75g glucose tolerance test. Imaging studies, including whole-body PET CT and abdominal CT, showed no evidence of suspected metastatic lesions, and the pancreas appeared normal on MRI. Endoscopic ultrasound could not be performed due to anatomical changes following the surgery. A selective arterial calcium stimulation test demonstrated a substantial over twofold increase in insulin concentrations in the gastroduodenal artery, splenic artery, and superior mesenteric artery. The patient received methylprednisolone 125 mg once daily but insulin levels remained elevated, exceeding 1000 uIU/ml, requiring continuous dextrose infusion. Octreotide therapy was initiated, resulting in an improvement in insulin levels to 589 uIU/ml, although discontinuation of dextrose fluid remained unfeasible. After consultation with the surgeon, a open subtotal pancreatectomy was performed, allowing discontinuation of dextrose on the second post-surgery day, with improved glucose and insulin levels. Histological examination showed three clusters of atypical cells with perineural infiltration. Immunostaining indicated negative synaptophysin and positive CK20, suggesting possible gastric cancer metastasis, consistent with prior gastrectomy findings. Moreover, an increased number of variable-sized Langerhans islets exhibited positive insulin and glucagon immunostaining within the islets, indicative of nesidioblastosis. Conclusion: Nesidioblastosis should be considered in cases of unexplained post-surgery hypoglycemia following metastatic gastric cancer treatment. Furthermore, given the potential link with metastatic gastric cancer, which may elude detection via imaging alone, aggressive surgical intervention should be contemplated as a nesidioblastosis treatment strategy. Presentation: 6/3/2024