In 2013, a working group of the World Health Organization (WHO) proposed a reformulation of the posttraumatic stress disorder (PTSD) diagnostic criteria for the upcoming 11th edition of the International Classification of Diseases (ICD‐11; Maercker, Brewin, Bryant, Cloitre, van Ommeren, et al., 2013). This study investigated the consequences of the proposed ICD‐11 PTSD symptom reduction on the prevalence of PTSD in children and adolescents. Prevalence rates of PTSD in a clinical sample of 159 traumatized children and adolescents were compared applying criteria according to the 4th edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM‐IV; American Psychiatric Association, 1994), the ICD‐10 (WHO, 1992), and the ICD‐11. The prevalence rate was 76.1% using DSM‐IV, 88.1% using ICD‐10, and 61.0% using ICD‐11. The use of the criteria proposed for ICD‐11 resulted in 27.1% less positive cases compared with ICD‐10 and 15.1% less positive cases compared with DSM‐IV. Our results showed that in a clinical sample of children and adolescents the prevalence of PTSD was significantly affected by the use of different diagnostic systems. This will constitute a major challenge for research and practice because, depending on the algorithm used, different groups of patients will be included in studies and different groups of individuals will be able to access medical care and therapy.