The purpose of this study was to compare serum metal ion concentrations (chromium and cobalt) in 3 groups, 2 with metal-on-metal articulations and a control metal-on-polyethylene group. Forty-six patients with good to well-functioning hips were recruited for the study. Serum ion levels of all patients were drawn, and Harris Hip Score, University of California, Los Angeles activity score, and radiographs were performed. Serum chromium concentrations were significantly lower in the large head group compared with the small head group (P = .013). There was no difference in the cobalt concentrations between the 2 groups (P = .087). There was a significant difference between both metal-on-metal groups when compared with controls for both chromium and cobalt levels (P = .0005 and P = .0004, control vs small; P = .001 and P = .0001, control vs large, respectively).