PURPOSE: The postoperative maxillary cyst (POMC) usually has an anterior wall defect of the maxillary sinus, and this make the evaluation of POMC by ultrasonogrphy. This study was to evaluate the ultrasonographic findings of POMC and to assess its accuracy in diagnosis. MATERIALS and METHODS: A prospective ultrasonographic evaluation was performed in twenty three patients (26 cases) with POMC. Twenty of them were confirmed by operation while three cases, by aspiration biopsy. Simple radiography and CT scan were performed in 17 and 11 patients, respectively. A 7.0 MHz linear probe was used in all of cases, and a 3.5 MHz sector probe was also additionally used in 10 cases. The positive sonography finding of POMC was a hypoechoic or anechoic mass with strong posterior wall echo. Various findings of ultrasonography were analyzed, and the sensitivity and specificity were calculated. RESULTS: Out of 26 cases, 23 (88%) were POMCs. 19 (83%) of 23 cases showed a hypoechoic or anechoic mass. 4 (17%) cases showed hyperechoic pyogenic cysts. The sensitivity, specificity and accuracy were 91%, 100% and 92%, repectively. CONCLUSION: Ultrasonography is an useful and non-invasive method for evaluating POMC.