To evaluate ultrasound (US) versus conventional plain film radiography (CPF) in the detection of pneumoperitoneum, 30 patients with postsurgical pneumoperitoneum and a control group of 22 patients were studied using US and CPF. Sonograms and radiograms were obtained while patients were supine and in left lateral decubitus. The two orthogonal plain films of the abdomen were acquired with a horizontal X-ray beam. The epigastric region and right hypochondrium were investigated with ultrasonography. Four experienced, blinded radiologists examined 160 sonograms and 104 radiograms. Statistical analysis yielded a sensitivity of 75.7% for radiography versus 86% for ultrasonography, a specificity of 89.2% versus 83.5%, an accuracy of 81.5% versus 85%, a positive predictive value of 90.2% versus 87%, and a negative predictive value of 76.2% versus 83.5%, respectively. US could therefore be considered a reliable alternative imaging technique in the detection of pneumoperitoneum.