There has been growing concern regarding the systemic and local effects of metal ions released from metal-on-metal (MOM) total hip replacements. There is limited data comparing the serum levels of metal ions after converting MOM to metal-on-polyethylene (MOP) bearings during revision total hip arthroplasty (THA). The purpose of this study was to report observed changes in the serum metal ion concentration following the revision of MOM bearings in THA. From Sept. 2010 to Nov. 2011, 23 patients with symptomatic MOM THA were revised to a MOP bearing. 10 patients had bilateral MOM THAs and underwent revision of the more symptomatic side only. All patients had normal renal function based on preoperative lab values. Serum levels of cobalt and chromium were tested prior to the revision, and were repeated between 6 weeks and 7 months postoperatively. Changes in levels of cobalt and chromium were analyzed with the Wilcoxon signed-rank test. Average cobalt serum levels were 30.6 μg/L preoperatively and 6.8 μg/L postoperatively. Average serum chromium levels were 19.7 μg/L preoperatively and 9.0 μg/L postoperatively. Both decreases were statistically significant (p < 0.0001). No patient had an increase in serum levels following the revision surgery. Both before and after the revision, patients with bilateral MOM THA exhibited higher blood levels of cobalt and chromium compared to patients with a unilateral MOM THA. Of those with unilateral MOM THAs, six patients (46%) had cobalt levels < 1.0 μg/L and four patients (31%) had chromium levels < 1.0 μg/L following revision. Serum levels of cobalt and chromium decrease following the conversion of a MOM to a MOP bearing surface. Cobalt levels appeared to decrease more rapidly than chromium levels at short term follow-up, the significance of which remains to be determined. Few patients had normal serum levels at early follow-up after revision.