A 64-year-old man was pointed out having a retroperitoneal tumor by abdomonal ultrasonography (US) on a medical checkup in 1993. There was a previous history of undergoing left nephrectomy at elsewhere 25 years before. After admission a fist-sized hard mass was palpated beneath the surgical scar, but no fluctuation nor movability was felt. Abdominal US revealed a well-defined round tumor in the left retroperitoneum, with hypoechoic surrounding region and nodular strong internal echo accompanied by acoustic shadow. CT visualized a well-defined round tumor which was not enhanced. Magnetic resonance imaging showed branch-like and concentric circular figure with a low intensity capsule on both enhanced T1 and T2, and internally low intensity on T1 and hetero intensity on T2 images. The patient was diagnosed as having retroperitoneal abscess due to retained surgical sponge. On March 29, 1993, the tumor was extripated. The tumor was covered with thick granulation and milk-white pus and tattered sponge were confirmed in the tumor. In diagnosing a tumor due to retained surgical sponge, medical history and imagings such as US, CT, and MRI were valuable.