Implementing public health surveillance at mass gatherings might help detect outbreaks or possible acts of biologic terrorism and enable prompt public health intervention. In July 2005, a daily syndromic sentinel surveillance system was implemented to monitor disease and injury among approximately 43,000 youths and adults attending a 10-day camping event held every 4 years by a national youth organization. Camp activities began on July 25, 2005, and included events such as mountain boarding, rappelling, and whittling. This report describes public health surveillance and response activities during the 10-day event and presents recommendations for health surveillance at large outdoor events. Public health surveillance should be implemented at mass gatherings to facilitate rapid detection of outbreaks and other health-related events and enable public health teams to respond with timely control measures.