The ultrasonic features in three cases and computed tomographic (CT) findings in one case of mucocele of the appendix are presented. This entity appeared cystic on sonography and may have high-intensity echoes within it. The wall was not thickened and did not contain calcification, unlike previous descriptions. On CT, the mass was of soft tissue density, while previously, it had been described as cystic. Although mucocele of the appendix has a more variable appearance on sonography and CT than previously reported, a correct preoperative diagnosis can be made in most cases. Barium enema examination or calcification in the wall or in the mass itself may be necessary to distinguish this entity from lymphoma if the lesion is of soft tissue density on CT.