A prospective longitudinal follow-up study (n = 59) of child and adolescent survivors of physical assaults and motor vehicle accidents assessed whether cognitive processes predicted posttraumatic stress symptomatology (PTSS) at 6 months posttrauma in this age group. In particular, the study assessed whether maladaptive posttraumatic appraisals mediated the relationship between initial and later posttraumatic stress. Self-report measures of PTSS, maladaptive appraisals, and other cognitive processes, as well as structured interviews assessing for acute stress disorder and posttraumatic stress disorder (PTSD), were completed at 2-4 weeks and 6 months posttrauma. PTSS and PTSD at 6 months were associated with maladaptive appraisals and other cognitive processes but not demographic or objective trauma severity variables. Only maladaptive appraisals were found to associate with PTSS/PTSD after partialing out initial symptoms/diagnosis and to mediate between initial and later PTSS. It was argued that, on this basis, maladaptive appraisals are involved in the development and maintenance of PTSS over time, whereas other cognitive processes (e.g., subjective threat, memory processes) may have an effect only in the acute phase. The implications of this study for the treatment of PTSS in youths are discussed.