The case: The patient is a 69-year old man. Ultrasonography (US) revealed a 47 x 35 mm space-occupying lesion (SOL) in the upper right quadrant of his abdomen. The internal echo pattern was homogeneous and more hyperechoic than the kidney cortex. The posterior echo was accentuated, and a well-defined border and a hypoechoic margin were observed. Color Doppler imaging detected no internal blood flow.After surgery, pathological examination of the mass lead to a diagnosis of fat necrosis covered with a fibrous capsule. Discussion: Concerning pseudotumoral fat necrosis, a qualitative diagnosis is made by differences in stage of disease at time of discovery. If, however, a mass with a fibrous cap and no internal blood flow is detected in the mesentery, the possibility of this kind of fat necrosis should be born in mind. Further, because surgical methods may vary depending on degree or range of adhesion, we suggest that it is more important to investigate existence, degree, or range of adhesion by utilizing the greatest advantage of ultrasound, namely the possibility of real-time observation of mass movement.