Air in the fissure for the ligamentum teres: new sign of intraperitoneal air on plain radiographs. In each of four patients in whom an acute spontaneous pneumoperitoneum developed, a vertically directed area of hyperlucency in the right upper quadrant was seen on radiographs of the abdomen obtained with the patient supine. This finding, which appeared in the absence of other characteristic signs of free air on plain radiographs and which, to the authors' knowledge, has not been previously recognized, represented intraperitoneal gas confined to the fissure for the ligamentum teres (FLT). The location of the hyperlucent area was confirmed with computed tomography or laparotomy in each patient. The distinctive configuration of air in the FLT is a subtle but reliable indicator of pneumoperitoneum. 