Insufficiency fracture of the superior part of the acetabulum after hemiarthroplasty has not been reported before. Here we report a case of dislocation after hemiarthroplaty due to insufficiency fracture of the acetabulum. In our patient, insufficiency fracture of the ilium immediately above the acetabulum produced obvious evidence of compression and collapse that led to dislocation of the outer head of the femoral component. This patient had subsequently been treated with a cementless socket inserted into the fractured acetabulum. We should remember the possibility of insufficiency fracture of the superior acetabulum after hemiarthroplasty, particularly in elderly patients suffering from chronic postoperative pain and disability.