Ultrasonic findings in 3 cases of perforated duodenal ulcer are described. The following common ultrasonic findings were noted: free gas in the abdominal cavity, a small amount of ascites and disappearance of gas shadow in the duodenal bulb. In two cases, fine echogenic substances probably derived from a mixture of gastric juice, bile, air and food residues were noted in the ascitic fluid. In one case, echogenic streak penetrating into the anterior wall of the duodenal bulb suggesting perforated ulcer was detected. These findings seemed to be useful for making a diagnosis of perforated duodenal ulcer on ultrasonic examination.