50 patients undergoing total hip replacement surgery with a titanium modular neck system were randomized to receive either a metal-on-metal bearing with a chromium-cobalt shell or a metal-on-polyethylene bearing with a titanium shell. The purpose of the study was to observe serum ion levels of chromium, cobalt, and titanium over 2 year follow-up. Titanium and cobalt levels were significantly increased within both groups after 2 years, while chromium levels only increased significantly for the metal-on-metal group. When comparing groups, the metal-on-metal group had significantly higher levels of chromium and cobalt at 2 year follow-up, while the metal-on-polyethylene group had significantly greater levels of titanium after 2 years.