45 patients scheduled for total hip arthroplasty were randomized to either a metal-on-metal or metal-on-polyethylene bearing surface. The purpose of this study was to compare whole blood concentrations of cobalt and chromium between the two groups, as well as evaluate and compare chromosomal damage in peripheral lymphocytes between groups. Follow-up was performed over 5 year postoperatively. Metal ion levels of both cobalt and chromium in whole blood samples were found to be significantly higher in the MoM group compared to the MoP group for the duration of follow-up. However, no significant differences in chromosomal damage - classified as either translocations, chromosomal breaks, or loss or gran of chromosomes - were observed between groups at 5 years postoperatively.