In this prospective randomized study, we investigate metal ion concentrations and clinical outcome 2 years after metal-on-metal (28 patients) or metal-on-polyethylene (26 patients) hip arthroplasty with 28-mm modular heads. Metal ion concentrations in patient serum were analyzed by high-resolution plasma mass spectrometry. The clinical outcome was almost identical in both groups with respect to the Harris hip score and the Medical Outcome Study Short Form-36; and on plain radiography, no signs of loosening occurred in any group. In the metal-on-metal group, concentrations of all investigated ions increased significantly when compared with preoperative values: cobalt, 15.3-fold (95% confidence interval [CI], 9.4-21.2); chromium, 5.2-fold (CI, 3.5-7.0); nickel, 2.1-fold (CI, 1.2-3.0); and manganese, 1.6-fold (CI, 1.3-2.0). In the metal-on-polyethylene group, a smaller but significant increase in the concentrations of cobalt and nickel occurred, whereas concentrations of the other ions did not change significantly.