In November 2001, a 29-year-old woman was admitted to the hospital because of dysphagia due to an apallic state caused by cerebral anoxia. Nutritional support was maintained by nasogastric tube feeding for approximately 3 months. For improvement of the body state maintenance and quality of life, a percutaneous endoscopic gastrostomy (PEG) was performed. Three weeks after the PEG, the patient had a wound infection and abdominal distension appeared. Marked pneumoperitoneum was confirmed by radiological examination. No signs or symptoms of peritoneal inflammation developed. A gastrografin study showing that the PEG tube was in the stomach appropriately was checked, and it was noted to be firmly in place without extravasation of contrast. After suspension of the tube feeding and tube opening to decrease intragastric pressure, intravenous hyperalimentation was performed. The pneumoperitoneum resolved within 7 days. Forty days after the PEG, tube feeding was resumed successfully. No recurrence of pneumoperitoneum developed and the patient has remained stable until the present time. The etiology of this finding probably occurs by insufficient fixation of the PEG, causing leakage of air through the gastric wall which enters the free peritoneal space. We recommend that the external binder should be kept 1 cm away from the abdominal skin after the gastrostomy fistula has formed and matured, and periodic rotation of the tube to verify that the internal bumper is free and sufficiently fixed to the gastric wall. In the case of abdominal distension after PEG placement, a X-ray examination and computed tomography (CT) scan with contrast medium would be helpful to ascertain pneumoperitoneum.