A blinded, retrospective study was performed to determine the value of supine 
abdominal radiographs in diagnosing pneumoperitoneum. Supine films from 44 cases 
of pneumoperitoneum were randomly interspersed among supine films from 87 
control subjects without free air, and the films were reviewed for the presence 
or absence of various signs of pneumoperitoneum, including Rigler's sign (gas on 
both sides of the bowel wall), the falciform ligament sign (gas outlining the 
falciform ligament), the football sign (gas outlining the peritoneal cavity), 
the inverted-V sign (gas outlining the medial umbilical folds), and the 
right-upper-quadrant gas sign (localized gas in the right upper quadrant). One 
or more of these signs were present in 26 cases (59%) of pneumoperitoneum, 
including the right-upper-quadrant gas sign in 18 cases (41%), Rigler's sign in 
14 cases (32%), and the falciform ligament and football signs in one case each 
(2%). Unfortunately, there were frequent errors in the interpretation of the 
right-upper-quadrant gas sign and Rigler's sign, with a total of 11 
false-positive cases (13%). Further analysis of the true-positive 
right-upper-quadrant gas signs showed that these gas collections were always 
triangular or linear with an inferolateral to superomedial orientation and, if 
triangular, a concave superolateral border. In the true-positive Rigler's signs, 
the bowel wall thickness ranged from 1 to 8 mm, whereas the false positives all 
had a bowel wall thickness of 1 mm or less. Proper interpretation of the various 
signs of pneumoperitoneum on supine films should lead to more accurate diagnosis 
of this condition.