To compare the sensitivity of CT with upright chest radiography for the detection of free intraperitoneal air, we compared the results of these examinations performed on trauma patients who had introduction of intraperitoneal air from diagnostic peritoneal lavage (DPL). Thirteen patients were studied by abdominal CT within 24 h after DPL. Upright chest radiography was performed prior to abdominal CT or less than 4 h after abdominal CT. All patients demonstrated free air on abdominal CT. Only 5 of 13 (38%) patients demonstrated free air on plain radiography. The amount of free air demonstrated on CT was quantified into three groups. Upright chest radiography in the minimal group (less than three 1 mm pockets of air) was totally insensitive (0 of 2) in detecting free air. Upright chest radiography in the moderate group (greater than three 1 mm pockets, but less than 13 mm diameter collection of air) was 33% sensitive (3 of 9). Upright chest radiography in the large group (greater than 13 mm collection of air) was 100% sensitive (2 of 2). Abdominal CT is clearly superior to upright chest radiography in demonstrating free intraperitoneal air in this clinical setting.