OBJECTIVE: To compare sex differences regarding the incidence of concussions among collegiate athletes during the 1997-1998, 1998-1999, and 1999-2000 seasons. DESIGN AND SETTING: A cohort study of collegiate athletes using the National Collegiate Athletic Association (NCAA) Injury Surveillance System; certified athletic trainers recorded data during the 1997-2000 academic years. SUBJECTS: Collegiate athletes participating in men's and women's soccer, lacrosse, basketball, softball, baseball, and gymnastics. MEASUREMENTS: Certified athletic trainers from participating NCAA institutions recorded weekly injury and athlete-exposure data from the first day of preseason practice to the final postseason game. Injury rates and incidence density ratios were computed. Incidence density ratio is an estimate of the relative risk based on injury rates per 1000 athlete-exposures. RESULTS: Of 14 591 reported injuries, 5.9% were classified as concussions. During the 3-year study, female athletes sustained 167 (3.6%) concussions during practices and 304 (9.5%) concussions during games, compared with male athletes, who sustained 148 (5.2%) concussions during practices and 254 (6.4%) concussions during games. Chi-square analysis revealed significant differences between male and female soccer players (chi(2)(1) = 12.99, P =.05) and basketball players (chi(2)(1) = 5.14, P =.05). CONCLUSIONS: Female athletes sustained a higher percentage of concussions during games than male athletes. Of all the sports, women's soccer and men's lacrosse were found to have the highest injury rate of concussions. Incidence density ratio was greatest for male and female soccer players.