Introduction: There have been reported adverse reactions in patients with large head metal-on-metal hip arthroplasty, therefore metal particle debris are a cause for concern. The aim of this study was to evaluate the release trend of metal ions and the correlated variables. Methods: 54 patients were prospectively enrolled into the study from 2004 to 2006. All patients had 36-mm metal head, Pinnacle acetabular component, Summit stem and Ultamet CoCr alloy liner (Depuy Inc.), and underwent clinical and radiological management. The haematic concentration of Cr-Co at 0, 6, 12, 24, 60 and 120 months after the implantation was analysed in a sub-cohort of 34 patients. Results: 10-year revision rate for each case was 8.9%. The average concentration of Co at 120 months was 3.12 μg/L (median 2.20) with 53% values >2 μg/L and 1 >7 μg/L. After 6 months the median levels of Cr and Co were higher than time 0 (p < 0.0001); no significant differences were found between 6 and 24 months; while Co had significant increase between 60 and 120 months (p = 0.002). Overweight, University of California at Los Angeles (UCLA) >9 and <50 years old patients have significantly higher circulating metal ions. Discussion: This is 1 of the few prospective studies on the argument. The revision rate is conforming to records. Our data shows ionic concentration is not predictive of revision or adverse reaction to metals. We described a 3-phase trend for Co probably due to the loss of integrity of the surfaces.