Purpose: We have diagnosed and treated perforated duodenal ulcers (PDU) with ultrasonography (US). As a result of our experience, we have obtained useful ultrasonographic findings regarding the diagnosis and treatment of PDU. Subject and method: We have experienced 24 PDU cases over a six-year and four-month period (May 2002 to June 2006), and have evaluated five useful ultrasonographic findings regarding PDU as described below: 1. Tow-way transmigration of liquid and air bubbles through the perforated duodenal wall. 2. Hyperechoic band penetration out through the duodenal wall. 3. Free air exiting the PDU. 4. Fluid exiting the PDU. 5. Liver covering over the PDU. Result and discussion: Of the total 24 cases, two-way transmigration was observed in 4 cases, hyperechoic band penetration was observed in 18 cases, and exiting free air and fluid was observed in all cases. In all cases of two-way transmigration of liquid and air bubbles, we could immediately diagnose the PDU. For the PDU cases regarding parts 2, 3, and 4 (as described above), upon analysis, a large majority provided useful ultrasonographic findings for diagnosing PDU. Of the 5 cases where the liver did not at all cover the PDU, two-way transmigration of liquid and air bubbles was observed in 4 cases, and operation was performed on all 5 cases. Of the 19 cases where the PDU was completely covered by the surrounding organs (liver comprised 16 cases, fatty organs such as liver teres, omentum etc. comprised 3 cases), non-operative treatment was selected for 18 cases. As a result, cases concerning PDUs covered by surrounding organs, mainly by the liver, received non-operative treatment, whereas non-covered PDUs received an operation. Conclusion: Ultrasonography is a useful for diagnosing and treating PDU.