OBJECTIVE: In sports concussion research, the importance of an individualized approach incorporating neuropsychological assessment data has been emphasized. This study examined the impact of acute signs of concussion on post-injury cognitive functioning using reliable change methodology in a sample of Finnish, elite-level, youth ice hockey players. METHODS: From a sample of 1,823 players (all male, 14-20 years old) who completed preseason baseline testing with the Immediate Post-Concussion Assessment and Cognitive Testing (ImPACT(R)) battery, two subgroups were identified. First, in total, 312 uninjured athletes, who completed baseline testing twice-1 year apart. The scores were contrasted to calculate reliable change indices (RCIs). Second, from a subsample of 570 athletes participating in an intensive follow-up arm of the project, the analysis included 32 concussed athletes. The RCIs were determined for the five ImPACT composite scores and used in identifying athletes with declined performance 3 days post-injury. RESULTS: Test-retest reliability ranged from .39 to .71. Athletes who had experienced an acute loss of consciousness, amnesia, or postural instability had increased odds for declines in two or more areas assessed by ImPACT (odds ratio = 7.67-8.00, p < .05). In contrast, acute disorientation or vacant look did not lead to cognitive change that met the reliable change threshold. CONCLUSIONS: The reliability coefficients and RCIs differed from those published earlier emphasizing the importance of national reference values. The presence of acute loss of consciousness, amnesia, or postural instability may indicate a more severe injury and predict the need for more intensive cognitive follow-up.