In this study, the types and costs of unintentional injuries among adolescents transitioning to adulthood are examined to provide age-appropriate prevention strategies. The data were collected in 2003, 2005 and 2007, in which a total of 273 (41%), 228 (39%) and 176 (33%) youths, respectively, reported to be having at least one serious injury. The leading types of injuries were sprains/strains, broken bones and bruises. Most injuries occurred while playing sports, falling/tripping, biking or rollerblading, mainly in recreation centres (>12-15%), schools (<27-9%), and workplaces (>2-14.5%). Most injuries were treated at emergency departments, walk-in clinics and health professional's offices (68-84%). Prevention included: doing nothing; being more careful; giving up the activity and rarely, rehabilitation or physiotherapy. The total direct cost of treatment was $471,498, (Canadian) at a mean direct cost of $775 per injury. Improved sports training and educational strategies targeted at subgroups of adolescents are needed to reduce the human and economic burden of injury.