Pneumoperitoneum secondary to gastrointestinal tract perforation is usually detected with plain abdominal films. Even when the examination cannot be performed in the upright position, free abdominal gas can be demonstrated with the accurate evaluation of supine radiographs. Some literature studies showed the value of ultrasonography and its higher diagnostic yield than that of plain abdominal films. However, in our experience on a series of 37 patients with surgically confirmed post-perforative pneumoperitoneum who underwent US first and radiography second, US had lower sensitivity than radiography (76% vs 92%, respectively). This was only partially compensated for by the detection of other signs and by better etiologic orientation. Therefore, we believe plain abdominal films to be still the method of choice to study the patients with suspected gastrointestinal perforation. Thus, US should be used in selected cases only--i.e., clinical conditions preventing radiographs from being performed correctly, persisting clinical suspicion with negative or questionable radiographic findings, the exclusion of other acute abdominal conditions, and finally the presence of pneumoperitoneum in the patients referred for different clinical reasons.