In a one-surgeon study the clinical and radiographic results of 30 cementless bipolar hip prostheses in 24 patients younger than 55 years were evaluated. Eleven noncoated prostheses (Noncoated Group) and 19 hydroxyapatite-coated prostheses (Hydroxyapatite Group) were compared after a mean followup of 10.4 years. The Harris hip score increased from a preoperative average of 41 points to 76 points at final followup (Noncoated Group, 70 points; Hydroxyapatite Group, 80 points). Thigh or groin pain was present in 15 patients (16 hips) (Noncoated Group, 55%; Hydroxyapatite Group, 53%). Radiographically, subsidence was the major problem at the noncoated prostheses (Noncoated Group, 91%; Hydroxyapatite Group, 5%), whereas osteolysis mainly was seen at the hydroxyapatite-coated prostheses (Noncoated Group, 18%; Hydroxyapatite Group, 89%). The obtained aseptic revision rate of 27% (Noncoated Group, 27%; Hydroxyapatite Group, 26%) is too high to use this implant in young patients. The large amounts of polyethylene wear debris generated by the bipolar system play an important role in this limited success. In the initially well-fixed hydroxyapatite-coated prostheses the sealing effect of a hydroxyapatite coating creates high concentrations of polyethylene in the limited joint space, resulting in massive proximal femoral osteolysis. Consequently, a hydroxyapatite coating introduces a new failure mechanism. Therefore, hydroxyapatite does not improve the outcome of a cementless bipolar hemiarthroplasty in the long-term.