Thirty-eight patients with a total of 45 primary uncemented bipolar hemiarthroplasties with smooth femoral components were observed for an average period of 5 years. Clinical results were excellent in 17, good in 15, fair in 10, and poor in 3 hips. Radiographs showed stem loosening and bipolar head migration in 12 and 2 hips, respectively. Endocortical femoral osteolysis developed in 26 hips (58%). Eight hips required revision surgery. All the revised hips showed circumferential wear of the polyethylene insert rims without substantial wear of the bearing surface. Histologic examination of the fibrous membrane around the stem showed many polyethylene particles that had been phagocytized by macrophages or surrounded by foreign body giant cells. No metal particles were observed at this site. The intrinsic functional mechanism of the bipolar endoprosthesis leads to persistent cyclic impingement of the stem neck on the rim of the bipolar head, producing polyethylene particles and frequent stem loosening by periprosthetic osteolysis. Unless this structural problem can be corrected and the durability of the prosthesis increased, the bipolar system should not be used for hemiarthroplastic surgery.