The authors submit case-histories of patients with an X-ray finding of pneumoperitoneum on a native X-ray of the abdomen. Air in the abdominal cavity was proved in the majority of cases also on laparotomy, however, the source of air was not found. The authors mention four cases from their own department. In the first female patient with a generalized gynaecological tumour pneumoperitoneum was diagnosed repeatedly before surgery. In the second case a female patient was involved with an inoperable pancreatic tumour; obstruction of the biliary pathways was resolved by an endoscopically inserted stent. In that case too the penumoperitoneum was repeatedly detected before surgery. The third patient with a generalized ovarian tumour, acute abdominal pain and pneumoperitoneum died on the second day after operation. Similarly as patient 2 on necropsy thrombosis of the portal vein was found. In the 4th case the finding of pneumoperitoneum was made after diagnostic technically difficult gastroscopy. This patient was not operated. Patients 1–3 were operated without evidence of perforation of the digestive tract. For illustration cases described in the literature are quoted. The causes of non-surgical pneumoperitoneum are classified according to the anatomical site of development. The author describes the assumed mechanisms of development of non-surgical pneumoperitoneum. In the discussion aspects are mentioned which decide on conservative or surgical treatment of pneumoperitoneum.

        Key words: acute abdomen – pneumoperitoneum – non-surgical pneumoperitoneum – causes – diagnosis – treatment