Objective: There is no universally agreed upon definition of cognitive impairment. Moreover, there are no empirically validated psychometric criteria for identifying cognitive disorders, including post-concussional disorder, set out in the World Health Organization’s International Classification of Diseases (10th edn) or the Diagnostic and Statistical Manual of Mental Disorders, 4th edn. The purpose of this study is to develop and evaluate new, evidence-based, psychometric criteria for defining cognitive impairment in athletes with sport-related concussion. Design: The neurocognitive test performances from an archival normative database of healthy youth and a sample of concussed high school football players were used to refine clinically and theoretically derived criteria for cognitive impairment. A clinical algorithm was developed for the following classification ranges: broadly normal, below average, well below average, unusually low and extremely low. Setting: Injured subjects were recruited from high schools in Pennsylvania. Subjects: Healthy adolescent boys (N = 341) between the ages of 13 and 18 years completed computerised neurocognitive testing. These subjects comprise the normative data for ImPACT. A sample of 125 high school football players completed testing within 5 days of sustaining a concussion. Outcome Measures: ImPACT is a brief computer-administered neurocognitive test battery that consists of six individual test modules that measure attention, memory, reaction time and processing speed. The four primary composite scores used in this study were verbal memory, visual memory, reaction time and processing speed. Results: The majority of normative subjects (73%) and a minority of concussed athletes (21%) were classified as broadly normal (x2 (1) = 103.1, p,0.0001). In contrast, 56% of concussed athletes and only 8.5% of normative subjects fell into the unusually low or extremely low classification ranges (x2 (1) = 123.3, p,0.0001). Conclusions: This study represents a first step towards identifying evidence-based criteria, with known false positive rates, for cognitive impairment associated with concussion.