The most frequent causes of unilateral limb swelling unrelated to trauma or surgery are deep venous thrombosis, chronic venous insufficiency, and primary lymphedema. Other important but less frequent causes include infection and neoplasm. Neoplasms may cause limb swelling, either by soft tissue enlargement or incidental compression of venous or lymphatic structures. Reported here is an unusual case of a patient with unilateral leg swelling and an inguinal mass presenting years after total hip arthroplasty. The unilateral leg swelling was caused by compression of the right common femoral vein by a synovial cyst arising from the hip joint. Although only 2 cases have been described in the literature, such cyst formation is not uncommon with loose acetabular components. The most apparent cause of cyst formation was polyethylene debris. Treatment efforts should be directed at the source of the debris intraarticularly.