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Background: The purpose of this paper is to study the dependence of Co levels in hair on Co levels in blood after metal-on-metal (MoM) hip replacement and prove the suitability of hair analysis coupled to blood analysis in the decision process regarding implant revision evaluation. Methods: Hair samples of 19 MoM patients having both well-functioning and malfunctioning implants and Co mass concentration levels in blood between 0.2 μg L-1 and 221.0 μg L-1 were included. A method based on inductively coupled plasma mass spectrometry was validated and used to measure the Co level in hair. Results: The Co mass fraction in the hair of patients ranged between 0.011 mg kg-1 and 0.712 mg kg-1. A correlation analysis showed a statistically significant positive correlation (r = 0.932, P < .001) between Co in the hair and that in the blood in the full-level range and a statistically nonsignificant positive correlation (r = 0.595, P = .091) in the low-level range. Conclusions: A correlation between the Co level in the hair and that in the blood exists when the latter is clearly above the 7 μg L-1 mass concentration threshold suggested for implant revision evaluation. The correlation disappears when the Co level in blood approaches or falls down the mass concentration threshold and that in the hair approaches or falls within the normal population range of 0.004-0.14 mg kg-1. Accordingly, clinicians could consider a hair analysis coupled to a blood analysis to assess the revision of malfunctioning MoM implants that release metals in patient’s body.