Two cases of well-differentiated retroperitoneal liposarcoma were reported. In both cases, sonography showed a large, well-demarcated, hyperechoic, homogenous retroperitoneal mass containing multiple fine hyperechoic lines. The hyperechoic lines changed their direction according to the utilized ultrasonic scanning methods as follows. With the linear scanning method, the lines ran parallel to the probe, and with the sector scanning method, the lines showed a concentric course around the probe, indicating that the hyperechoic lines were positioned vertically to the ultrasonic beam. We named this phenomenon "wind-and-wave sign." These "waves" (hyperechoic lines) changed their form with the direction of the prevailing "wind" (scanning method). Histologically, numerous fine fibrous septa developed in the lipomatous tumor tissue. Sonographically, the echogenicity of the object depended largely on the angle between the ultrasonic beam and the object. Furthermore, the direction of the ultrasonic beam changed according to the scanning method employed (linear versus sector). Therefore, the distribution of the hyperechoic lines, positioned vertically to the ultrasonic beam, changed according to the scanning method. This wind-and-wave sign is characteristic of lipoma or well-differentiated liposarcoma, and is considered useful for making a preoperative diagnosis.