Background: Hyperinsulinemic hypoglycaemia (HH) after Roux-en-Y gastric bypass (RYGB) is rare. Patients typically present with post-prandial hypoglycaemia >1 year after surgery and once weight loss has plateaued. Despite multiple treatment options, the management of these patients remains challenging. Clinical case: A 31-year-old female was referred for bariatric surgery with a BMI of 41.4 kg/m2. In the year preceding the referral to bariatric surgery she attended tier 3 services and managed to lose 29% of her body weight with life style intervention. She was known to have Type 2 diabetes, which was now well controlled with metformin (HbA1c 34 mmol/mol) following this weight loss. Twelve months after undergoing RYGB, her BMI had reduced to 25.5 kg/m2 and she presented with excessive sweating, headaches, palpitations and hunger. At this point she had lost 56% of her total body weight over a 30-month period. Two 16-h fasting tests did not elicit any proven hypoglycaemia. She was admitted as an inpatient for a 72-h fasting test, during which the lowest glucose level was 3.9 mmol/l, which was associated with insulin levels of <10 mU/l and C-peptide of 284 pmol/l. An endoscopic ultrasound showed no evidence of pancreatic tumours. A mixed meal test was then performed which showed hypoglycaemia (glucose 2.6, and 2.8 mmol/l) with inappropriate levels of Insulin (51 mU/l) and C-peptide (1056 pmol/l) 30 min post-prandial. She remained highly symptomatic despite dietary measures (small frequent meals, avoiding simple carbohydrates), and treatment with Acarbose. Hypoglycaemic events remained frequent despite a very low carbohydrate diet (<30 g per day). A restrictive dietary intervention using a ketogenic diet was thus initiated. The patient’s symptoms resolved entirely when carbohydrate intake was reduced gradually to 10–12 g of carbohydrate per day. Conclusion: Ketogenic dietary intervention might dramatically improve the situation in patients with post-RYGB HH that failed to respond to other measures or wish to avoid pharmacological intervention.