Case report: identification of the transverse mesocolon and root of small bowel mesentery; a new sign of pneumoperitoneum.

A new sign of pneumoperitoneum was observed in a patient with perforation of the ascending colon. The presence of free air in the peritoneal cavity allowed identification of the transverse mesocolon and the root of the small bowel mesentery on plain abdominal radiographs obtained in the supine and in the prone position. This sign may be an aid in the diagnosis of pneumoperitoneum.