OBJECTIVE: To determine the incidence of concussion in high school rugby players and compare the findings with the relevant published literature. DESIGN AND SETTING: Prospective data collection in one US high school rugby program. SUBJECTS: Two teams followed for 3 years. MEASUREMENTS: Injury rate, injury severity, and time withheld from competition. RESULTS: Seventeen concussions were recorded, accounting for 25% of all reported injuries. The incidence rate for concussion was 3.8 per 1000 athlete-exposures (95% confidence interval, 2.0-5.7) or 11.3 per 100 player-seasons (95% confidence interval, 5.9-16.7). Of the 17 concussions, 14 were Cantu grade 1, 2 were grade 2, and 1 was grade 3. Concussions accounted for 25% of all days lost from rugby participation due to injury. CONCLUSIONS: The incidence of concussion in this study was higher than previously reported in other studies. Methodologic limitations and administrative rules that required suspension for injured players may have suppressed reporting in previous epidemiologic studies. The incidence of concussion in rugby is probably much higher than previously suggested.