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Although polyethylene wear-induced osteolysis is a common complication of hip arthroplasty, extensile osteolysis developing into a large granulomatous thigh mass at a site distant from the joint is rare. We report a case of a thigh pseudotumor 25 years after cemented bipolar hemiarthroplasty, in which x-rays revealed a radiolucent line around the stem at the proximal site only, not at the diaphysis of the femur. We initially suspected a real tumor because it had a unique appearance, as if the mass resorbed the posterior cortex of the femur, and it was located at a site distant from the proximal osteolytic lesions. We clearly showed the existence of a connection between the thigh mass and the joint space by performing intra-articular injection of contrast medium with continuous pressure. It seemed that polyethylene wear particles were transported distally along the stem-cement interface by fluid pressure, and an osteolytic reaction against polyethylene wear particles had occurred at the posterior middle third of the stem where the cement mantle was nonuniform and polyethylene particles first came into contact with the bone. Our findings suggest that nonuniform cemented prosthesis with osteolysis, even if it is low grade in a limited area, carries the risk of extensile osteolysis with asymptomatic development of an extra-articular granulomatous mass. [ABSTRACT FROM AUTHOR]