Diagnosis via x-ray film had so far been the only way to prove the existence of free air in the abdominal cavity and hence the presence of gastrointestinal perforation. However, it is possible to establish the existence of free air in the abdomen sonographically provided a suitable examination method is employed. A characteristic feature is represented by pronounced pre-hepatic echoes with multiple echoes or sound shadow phenomenon. Experimentally it was possible to identify 1 ml air in the abdomen of an ascites patient both by x-ray examination and by sonography with equal certainty. The clinical usefulness of the ultrasound method was scrutinized. Of 10 patients with gastrointestinal perforations, 9 were recognized by sonography, including roentgen-negative cases; in one case only, sonography yielded a false negative result. Sonographic differential diagnosis is more versatile and differentiation can be practised by the highly experienced investigator only. The proof of the existence of free air in the abdomen represents a considerable extension of the scope of sonographic emergency-case diagnosis.