Recent work suggests that repetitive subconcussive head impacts may contribute to long-term neurodegeneration; however, the risk thresholds for subconcussive injury are unknown. It was hypothesized that the number and severity of head impacts could quantify the risk of developing abnormal neurophysiology. Twenty-one high school boys (ages 14 to 18) participating in varsity and junior varsity football were evaluated over the course of 1 football season (13 games) using a combination of the ImPACT, functional magnetic resonance imaging (fMRI), and head impact telemetry. The number and severity of head impacts throughout the football season were subsequently compared with the fraction of players flagged by either the ImPACT or fMRI before, during, and after the competition season. Twelve of 22 (54.5%) in-season tests of asymptomatic participants had a minimum of 1 composite ImPACT score flagged. For the fMRI analysis, 16 (72.7%) of the 22 in-season sessions were considered flagged. Seven assessments were flagged by both the ImPACT and fMRI. Larger numbers of hits appeared to correspond with a larger fraction of players being flagged by at least fMRI or the ImPACT. A substantial number of clinically asymptomatic athletes exhibit significant neurophysiological changes in-season. Furthermore, the number of head impacts is a risk factor for the development of neurophysiological changes. [Athletic Training & Sports Health Care. 2014;6(x):xxxxxx.] [Query #2: Reduce to 150 words.] The exact cause of long-term neurocognitive complications following participation in contact athletics is unclear, and it has been suggested that neurological deterioration is the result of a combination of concussive and multiple subconcussive impacts.1 Although the current diagnosis criteria for concussion relies heavily on the presence of symptoms such as headache, loss of consciousness, or slowed reaction time,2 subconcussive blows do not result in acute clinical symptoms. As a result, subconcussive blows are difficult to study, and research regarding their effects has only recently appeared in the literature. In an initial study, Talavage et al3 demonstrated that asymptomatic, nonconcussed high school football players could exhibit measurable changes in neurocognitive performance and neurophysiology, as measured by the Immediate Post-Concussion Assessment and Ms Breedlove and Dr Leverenz are from the Department of Health and Kinesiology, Mr Breedlove and Dr Nauman are from the School of Mechanical Engineering, Dr Robinson, Ms Poole, Mr Rasmussen, and Dr Talavage are from the Weldon School of Biomedical Engineering, and Mr King and Mr Rosenberger are from the School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana. Ms Poole is also from the Department of Basic Medical Sciences, and Dr Nauman is also from the Weldon School of Biomedical Engineering and the Department of Medical Sciences, Purdue University. [Query #1: OK as edited?] Received: April 16, 2013 Accepted: January 22, 2014 Posted Online: This work was supported by grants from the Indiana State Department of Health Spinal Cord and Brain Injury Research Fund, General Electric Healthcare, and through the National Science Foundation and National Defense Science and Engineering Graduate Fellowships. The authors thank Dr. Gregory G. Tamer, Jr, for his assistance with data collection. The authors have disclosed no potential conflicts of interest, financial or otherwise. Address correspondence to Eric A. Nauman, PhD, 585 Purdue Mall, School of Mechanical Engineering, Purdue University, West Lafayette, IN 47906; e-mail: firstname.lastname@example.org. doi:10.3928/19425864-2014xxxx-xx ATSHC_OAR_NAUMAN.indd 1 4/17/2014 3:08:02 PM 2 Copyright © SLACK Incorporated Breedlove et al Cognitive Test (ImPACT; ImPACT Applications, Inc; Pittsburgh, Pennsylvania) and functional magnetic resonance imaging (fMRI). Breedlove et al4 subsequently noted correlations between these measured changes and the number of subconcussive blows sustained throughout the season. Furthermore, these correlations were also found to exist for symptomatic athletes, suggesting that neurophysiological changes are linked to cumulative, subconcussive impacts. Given the possibility that repetitive, low-magnitude head impacts are linked to altered cognitive function, the purpose of this article is to use existing data from an ongoing research study to evaluate the possibility of quantifying the risk of developing abnormal neurophysiology due to repetitive subconcussive impacts to the head. We hypothesized that the number of head impacts sustained during the season would be predictive of neurological impairment. These data would be valuable to clinicians in making informed decisions about the risk of repetitive head impacts.