Recent studies suggest that the tapered interface between stem and femoral head may be a substantial source of cobalt and chromium ion release after metal-on-metal (MOM) total hip arthroplasty (THA). This study compared patient ion levels after MOM hip resurfacing (HR) and MOM THA performed with identical acetabular components. 110 HRs were compared with 22 THAs. All had well-oriented components, unilateral implants, and serum ion studies beyond one year post-operatively. The HR group’s median cobalt value was 1.11 μg/L vs. 2.86 μg/L for the THA patients. The HR group’s median chromium value was 1.49 μg/L vs. 2.94 μg/L for THA. Significantly higher THA ion levels suggest a source of ions other than the MOM bearing itself.