We report two cases of aneurysm of the external iliac artery after arthroplasty of the hip. In each case the patients suffered from severe, seropositive, rheumatoid arthritis, had been treated with oral corticosteroids and had defects in the acetabular floor which were complicated by sepsis. In these circumstances bleeding from the wound in the hip should be investigated by immediate arteriography with anteroposterior and lateral views. Though vascular injury during operations on the hip is rare, recognition is important as safe and satisfactory treatment can be achieved. In the surgical management of these cases the following points should be noted: an alternative blood supply to the limb must be established using separate surgical incisions; to reduce the risk of sepsis these incisions should be closed and dressed before exploring the aneurysm; the aneurysmal vessel must be isolated and ligated, no attempt being made at primary repair; the aneurysm should be opened longitudinally to avoid damaging the femoral nerve which overlies it; and all foreign material should be removed from the hip.