In this study, we determined the long-term prevalence of posttraumatic stress disorder (PTSD) in children and adolescents after accidental injury and gained insight into factors that may be associated with the occurrence of PTSD. In a prospective longitudinal study, we assessed diagnosed PTSD and clinically significant self-reported posttraumatic stress symptoms (PTSS) in 90 children (11–22 years of age, 60% boys), 2–4 years after their accident (mean number of months 32.9, SD 6.6). The outcome was compared to the first assessment 3 months after the accident in 147 children, 8–18 years of age. The prevalence of PTSD was 11.6% at first assessment and 11.4% at follow-up. Children with PTSD or PTSS reported significantly more permanent physical impairment than children without. Children who completed psychotherapy had no symptoms or low levels of symptoms at follow-up. Given the long-term prevalence of PTSD in children following accidents, we recommend systematic monitoring of injured children. The role of possible associated factors in long-term PTSS needs further study.