A case of retained surgical sponge was imaged by ultrasound and computed tomography (CT). Sonography revealed a hypoechoic mass with areas of high echoes and acoustic shadowing. An experiment revealed that the high echoes were attributed partly to the presence of numerous interfaces of sponges. The sonographic detection of a mass with high echoes casting acoustic shadows should alert radiologists to the possibility of retained surgical sponges even if there is no gas or calcification on CT scans.