100 patients undergoing a total hip arthroplasty without cement for unilateral osteoarthritis or unilateral osteonecrosis were randomized to be treated with a metal-on-metal articulation or with a ceramic-on-polyethylene bearing. Blood samples were taken multiple times over a 5 year period and serum cobalt concentrations measured. In the metal articulation group, median cobalt levels were 1 μg/L at 1 year postoperatively and 0.7 μg/L at 5 years. The median cobalt levels in the ceramic-on-polyethylene group were below detection limits at all times. The serum cobalt concentrations did not reflect a so-called run-in wear period of the metal-on-metal articulations.