A 43-year-old woman was referred to the obesity clinic due to neuroglycopenic symptoms caused by an insulinoma 2 years after a sleeve gastrectomy. In March 2020, 2 years after LSG was performed, the patient developed neuroglycopenic symptoms including short-term memory loss, lingual nerve paresthesia, and nonspecific visual alterations predominantly during the morning in a fasting state. These symptoms were suppressed with food intake. Two months later, she visited a physician who documented fasting plasma glucose of 27 mg/dL, and in June 2020, the symptoms occurred more frequently, and she gained 14 kg. In the beginning, late dumping symptoms were suspected, but in September 2020, fasting plasma glucose of 30 mg/dL was documented, so she was hospitalized for the evaluation of hypoglycemia in a 72-h supervised fast test. She had baseline plasma glucose of 67 mg/dL, nonsuppressed insulin of 16.4 IU/mL, and C-peptide of 3.64 ng/mL. In the first hour after initiation, she developed Whipple’s triad symptoms, and her lab results detected plasma glucose of 38 mg/dL, insulin of 25.9 IU/mL, and C-peptide of 4.31 ng/mL. Thus, it was decided to stop the protocol and initiate 1000 mL of 20% glucose solution in 12 h. In 2002, the patient was diagnosed with obesity and dyslipidemia (high triglycerides and cholesterol with low HDL) and treated with improvements in diet, physical activity, and statins without weight control. In 2016, a gastric balloon was placed, and although her body mass index (BMI) in 2018 was 34.4 kg/m2, LSG was performed. The patient had no specific personal or family history. After LSG, the patient weighed 74 kg, and her BMI was 32 kg/m2. The physical examination showed no obvious cardiovascular or respiratory abnormalities. The abdomen was soft, and the only sign was the presence of postsurgery scars. Upon hospitalization prior to the surgery, the patient’s hemoglobin A1c level was 4.8% (normal range: < 5.7%). The C-peptide value was normal at 3.64 ng/mL (1.1-4.4 ng/mL), and insulin was mildly elevated at 16.40 µUI/mL (3.21-16.30 µUI/mL). Lipid levels indicated dyslipidemia with total cholesterol of 224 mg/dL and LDL-c of 142.8 mg/dL. Other biochemical parameters were normal and only an iron deficiency anemia was documented. Thyroid function was normal, with TSH 2.46 µUI/mL (0.27-4.20 µUI/mL), FT4 1.06 ng/dL (0.93-1.70 ng/dL), and cortisol level 15.04 ug/dL (3.70-19.40 µg/dL), all within the normal range. Computed tomography (CT) demonstrated the presence of a focal asymmetric reinforcement area in the head of the pancreas. Endoscopic ultrasound showed the presence of a tumoral lesion in the pancreas in close proximity to the main pancreatic duct and splenomesenteric confluence without evidence of invasion.