There are a limited number of epidemiological studies that have focused on trauma exposure and prevalence of posttraumatic stress disorder (PTSD) in representative general population samples of adolescents, especially outside of the United States. We therefore aimed to assess the lifetime prevalence of traumatic events (TEs) and current prevalence of PTSD, and to examine demographic risk factors for TEs and PTSD in a representative sample of adolescents. Data were collected by a school survey among a sample of 6,787 9th-grade students in Switzerland. Roughly 56% of the adolescents (females 56.6%; males 55.7%) reported having experienced at least 1 TE. Non-Swiss nationality (OR = 1.80), not living with both biological parents (OR = 1.64), and lower parental education (OR = 1.18) were associated with a higher risk of trauma exposure. The current prevalence of PTSD according to the Diagnostic and Statistical Manual of Mental Disorders (4th ed., text rev.; DSM-IV-TR; American Psychiatric Association, 2000) criteria was 4.2% (females 6.2%; males 2.4%). Female gender (OR = 2.70), not living with both biological parents (OR = 1.47), lower parental education (OR = 1.51), and exposure to multiple TEs (OR = 9.56) were significant risk factors for PTSD. Results suggest considerably high rates of TEs and PTSD among adolescents. Intervention efforts must be intensified to reduce trauma exposure and treat PTSD.