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Zemper ED: Two-year prospective study of relative risk of a second cerebral concussion. Am J Phys Med Rehabil 2003;82:653–659.To prospectively measure the relative risk of cerebral concussion among those with a history of concussion compared with those having no previous concussions by using a population of high school and college football players.A representative national sample of high school and college football players was followed for two football seasons over a 2-yr period (1997–1998) as part of a national football injury surveillance project. There were a total of 15,304 player-seasons and over 1 million athlete-exposures to the possibility of injury in practices and games; 975 of the player-seasons (6.4%) had a history of concussion in the previous 5 yr.There were 572 concussions recorded, 161 among those with a history (16.5%) and 411 among those with no history (2.9%). Relative risk for individuals with a history of concussion is 5.8 times greater than for individuals with no history (95% confidence interval, 4.8–6.8).This large prospective cohort study indicates the risk of sustaining a cerebral concussion is nearly six times greater for individuals with a history of concussion than for individuals with no such history.