Context. Diabetic pseudoperitonitis is a very rare complication of the type 1 diabetes mellitus and it is associated with a severe ketoacidosis. The exact pathogenesis of the status is still unclear, the typical presentation is an acute abdomen by the patient. To confirm the diagnosis, it is necessary to make examinations, which exclude other possible reason of an acute abdomen by the patient (laboratory tests, abdominal ultrasound or a CT scan). Case presentation. A 46-years old man was admitted to the hospital wih a history of a 10 days epigastric pain. Laboratory tests, abdominal ultrasound, CT scan and upper endoscopy were performed, the reason of the pain remained unclear. Because of the peritoneal signs at the first day of the hospitalisation an acute surgery was indicated, without any pathology at the laparoscopy. A severe metabolic acidosis was recognized only after the surgery, the initial hypoglycaemia rose up after giving a total parenteral nutrition to the patient. The increase of the glycaemia, the severe metabolic acidosis with glycosuria and ketonuria, and the elevation of the glycated haemoglobin brought us to the diagnosis of the new onset of the diabetes. Conclusion. Diabetic pseudoperitonitis with the picture of an acute abdomen can occur as a first manifestation of the diabetes. Thinking of this rare complication and recognising it can avoid unnecessary acute surgery by the patient.