Introduction: Metformin may rarely cause lactic acidosis in patients with predisposing factors of acidosis or tissue hypoxia, like acute renal or heart failure, liver failure, dehydration, alcohol consumption or serious infection. Mortality may approach 50% in these cases. Case description: A 70-year-old lady came to the emergency unit because of vomits and diffuse abdominal pain. Five days ago, she had visited our hospital for the same reason, with normal findings on physical and laboratory examination. Her medical history included diabetes mellitus under metformin/vildagliptin and dementia. The patient was confused and dis- oriented, afebrile, oliguric, with tachypnea and diffuse abdominal tenderness. Pressure was 130/70mmHg. Blood gases revealed severe lactic acidosis (lactate 4 15mmol/L), pH 1⁄4 6.84, PCO2 1⁄4 7mmHg, pO2 1⁄4 133mmHg, gluco- se1⁄457mg/dL, HCO3o3mmol/L. Abnormal laboratory tests included creati- nine1⁄45.3mg/dL, urea1⁄4152mg/dL, WBC1⁄417000/mL, hemoglobin1⁄412.3gr/dL, sodium 1⁄4 133mmol/L, potassium 1⁄4 4.4mmol/L, ESR 1⁄4 43mm/h. Chest x-ray, abdominal ultrasound (to exclude obstructive nephropathy) and echocardio- graphy were normal. Results and conclusions: Thepatientreceived400mLbicarbonate4.8%, aggressive hydration, dopamine (diuretic dose) and 160mg furosemide. Because of clinical deterioration she underwent hemodialysis. She was treated, according to guidelines, as for severe sepsis with meropenem. Blood and urine cultures were negative. On 1st day, ECG showed ischemic lesions, which resolved with nitrates. Abdominal CT was normal. She remained afebrile after the 1st day (low grade fever). Overall, the patient underwent three hemodia- lysis sessions (resistant severe lactic acidosis, low bicarbonates). On 2nd day, she was well oriented. She was discharged 8 days later with urea1⁄459mg/dL and creatinine1⁄41.6mg/dL. After 20 days, creatinine was 1mg/dL. Take-home message: Metformin may be a cause of severe lactic acidosis, disproportionate to the degree of renal failure, in patients with previous normal renal function and acute dysregulation. Hemodialysis is a lifesaving therapeutic intervention in these patients.