Hyperglycaemic hyperosmolar state (HHS) may occur in young patients with type 1 and type 2 diabetes and in infants with hyperglycaemia. Hyperglycaemic hyperosmolar state is characterised by extremely high glucose concentration, which, by increasing osmotic diuresis, intensifies dehydration. Hyperglycaemic hyperosmolar state criteria include the following: plasma glucose > 600 mg/dl, venous pH > 7.25, sodium bicarbonate > 15 mmol/l, slight ketonuria, plasma osmolality > 320 mOsm/kg, and impairment of consciousness (aggression, unconsciousness, convulsions). We describe the case of a 13-year-old patient with severe obesity (at presentation body mass > 120 kg, BMI > 40 kg/m2), who developed HHS (glycaemia 647 mg/dl, pH 7.18, pCO2 96.5 mmHg, BE – 5.0 mmol/l, HCO3 35.2 mmol/l; Na 167 mmol/l, plasma osmolarity 370 mOsm/kg) in the course of pneumonia and newly diagnosed type 2 diabetes (HbA1c 15.5%, C-peptide 2.63 ng/ml). In the follow-up, due to the hypoglycaemia, insulin was discontinued, metformin was administered at a dose of 2 g/day, with a further reduction to 500 mg/day, together with physical rehabilitation and a low-calorie diet. Weight reduction during 6 months of observation was approximately 37 kg. Due to breathing disorders occurring at night, the girl still needs breathing assistance (CPAP).