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To examine neurofilament light (NF-L), a microtubule-associated protein widely regarded to be central nervous system (CNS) specific, concentrations in serum prior to and following a season of contact (American football) and non-contact (cross-country running) in elite, collegiate-aged athletes.Prospective cohort.Laboratory.13 male American football athletes (19±1.2 years) and 11 cross-country runners (20±1.9 years).Pre- and post-season blood samples were obtained from all participants and spun at 2000 g for 10 minutes to extract serum. NF-L was detected through the isolation of individual immunocomplexes on pragmatic beads using standard ELISA reagents. The beads were then trapped in single-molecule, femtolitre-sized wells, allowing for a digital readout of each individual bead. The digital nature of this technique allows an average of 1000x sensitivity increase over conventional assays.Analysis of pre- and post-season serum samples provide NF-L concentrations.Serum samples from American football players showed a significant increase (p=0.04) from pre-season NF-L serum concentrations (95% CI: 6.51 – 9.19 pg/mL) to post-season (95% CI: 7.08 – 11.70 pg/mL). Serum NF-L concentrations in cross-country runners showed no change (p=0.57) from pre-season (95% CI: 5.05 – 6.94 pg/mL) to post-season (95% CI: 5.05 – 6.88 pg/mL).Experiencing repetitive subconcussive head-trauma throughout a season of American football elevates serum NF-L concentrations. This finding indicates multiple head-impacts may lead to CNS axonal damage.None.