Perigraft seroma usually occurs both polyester and polytetrafluoroethylene (PTFE) graft which are placed superficially for axillofemoral and femorofemoral bypasses, while it is a rare complication of conventional abdominal aortic and iliac arterial aneurysm repair. The cause of the seroma has not been elucidated, and several hypotheses have been proposed such as immunologic response to graft materials, discharge of serous fluid through the graft wall, and so on. The seroma sac occasionally increases their size finally leading to rupture. The treatment of perigraft seroma has not been established so far; there have been various recommended procedures including aspiration, graft removal followed by other material graft replacement, cessation of antithrombotic drugs, and careful observation. We report two cases of perigraft seroma after conventional aortoiliac aneurysm repair with a knitted polyester graft via left pararectal retroperitoneal approach, which were gradually shrinking by theirselves.