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This study aims to investigate the role of microRNAs in early and late assessments of concussed athletes. Our hypothesis is that expression of certain microRNAs is altered following concussion, and these microRNAs have the potential to become novel biomarkers for early detection and monitoring of concussion.Prospective observational case control study.University research centre, single centre.15 concussed athletes divided into 2 groups: Group A (n=9, assessed within a week post-concussion, median 4 days) and B (n=6, assessed over 2 weeks, median 31 days); and 8 age-matched healthy volunteers. Inclusion criteria: male/female elite athletes in contact sports (aged 18-40) who suffered concussion(s) in the games, and being symptomatic with normal neuro-radiological findings at enrolment.Levels of microRNAs (miR-21, miR-335, miR-425-5p, and miR-502) were measured in blood samples obtained from all participants.miR425-5p was significantly down-regulated in Group A compared with Group B and healthy volunteers (grouping effect p=0.014; post hoc tests p=0.009 and 0.028, respectively). The level of miR425-5p correlated with the interval between assessment and concussion (r2=0.88, p<0.001). No differences were found in other microRNA assessed between the groups.miR-425-5p is down-regulated in the early phase following sport concussion, and reaches normal levels in the late period, suggesting that it may be a unique microRNA signature for concussion. This makes miR-435-5p a promising candidate biomarker for early evaluation of concussion and monitoring of its recovery. Further longitudinal studies are on-going to establish the time course of miR-425-5p normalisation after concussion. None.