Background: Admission to the Paediatric Intensive Care Unit (PICU) represents a traumatic and potentially life-threatening event that may have psychological as well as physical repercussions for children. The PICU presents a variety of unique stressors that place children at particular risk of developing elevated post-traumatic stress symptoms (PTSS) or posttraumatic stress disorder (PTSD). However, this cohort has received much less attention than other paediatric patients and parents of PICU patients. Preliminary research suggests that the psychological impact of PICU admission on children may be substantial, yet significant gaps remain in our understanding of the prevalence, aetiology and course of PTSS in this cohort. Further research is crucial, as in the short-term, these symptoms can result in high levels of distress and interference with daily functioning and in the long term, may lead to emotional dysfunction, adverse physical health outcomes and poorer health-related quality of life. Objectives: This study aimed to (1) describe children’s subjective experiences of PICU; (2) document the prevalence and course of posttraumatic stress in children up to 6 months post-discharge; (3) explore the presentation of PTSD in children 6 months post-discharge and identify the most sensitive and valid approach to diagnosis in this cohort; (4) identify the relative importance of premorbid, peri-trauma and cognitive factors associated with children’s acute and chronic posttraumatic stress; and (5) validate a widely used PTSS assessment measure (CRIES-13; Children’s Revised Impact of Event Scale) for use in this cohort. Methods: Families of 95 children (aged 6-16) were recruited consecutively from the Royal Children’s Hospital PICU, Brisbane, Australia for a prospective longitudinal cohort study. Assessments were conducted with children and parents 2-4 weeks and 6 months post-discharge to assess medical, demographic and premorbid factors, children’s recall of PICU and children’s current posttraumatic stress symptomatology. Results: Most children remembered aspects of their PICU admission, although few recalled their entire stay clearly. Children’s positive recollections of PICU were most commonly related to nurses and staff. Negative recollections were mostly related to pain and invasive procedures. Most children were psychologically resilient following PICU admission. However, up to 40% (30/75) of children reported elevated PTSS 2-4 weeks after discharge and up to 36% (20/55) reported elevated PTSS at 6 months. Overall, mean PTSS did not resolve over time but children displayed distinct individual recovery patterns. An alternative diagnostic PTSD algorithm, PTSD-AA (excluding Criterion C3), was identified as the most sensitive algorithm in identifying children with high symptomatology and functional distress. More than one in four children fulfilled these criteria 6 months post-discharge. Few premorbid or trauma characteristics were associated with PTSS but the addition of a range of cognitive factors including affect, processing/encoding, memory quality and behavioural avoidance accounted for a large proportion of additional variance in children’s acute (p<.01) and persistent PTSS (p<.001). Finally, this study also validated the use of a widely-used measure of concurrent PTSS (CRIES-13) in the PICU cohort, although modifications are required when using thresholds to classify children with “probable PTSD”. Conclusions: This research provides a significant contribution to our understanding of the impact of PICU admission on children. The results have important implications for assessment, diagnosis and intervention and it is recommended that screening and preventative strategies are incorporated into the model of care for PICU patients. This study also provides a number of recommendations for future research in this high-risk cohort, with the aim of reducing the prevalence of PTSS/PTSD and optimizing children’s psychological and physical recovering following PICU admission.