The Children's Post-Traumatic Cognitions Inventory (CPTCI) is a self-report questionnaire that measures maladaptive cognitions in children and young people following exposure to trauma. In this study, the psychometric properties of the CPTCI were examined in further detail with the objective of furthering its utility as a clinical tool. Specifically, we investigated the CPTCI's discriminant validity, test-retest reliability, and the potential for the development of a short form of the measure. Three samples (London, East Anglia, Australia) of children and young people exposed to trauma (N = 535; 7-17 years old) completed the CPTCI and a structured clinical interview to measure posttraumatic stress disorder (PTSD) symptoms between 1 and 6 months following trauma. Test-retest reliability was investigated in a subsample of 203 cases. The results showed that a score in the range of 46 to 48 on the CPTCI was indicative of clinically significant appraisals as determined by the presence of PTSD. The measure also had moderate-to-high test-retest reliability (r = .78) over a 2-month period. The Children's Post-Traumatic Cognitions Inventory-Short Form (CPTCI-S) had excellent internal consistency (α = .92), and moderate-to-high test-retest reliability (r = .78). The examination of construct validity showed the model had an excellent fitting factor structure (Comparative Fit index = 0.95, Tucker-Lewis index = 0.91, Root Mean Square Error of Approximation = .07). A score ranging from 16 to 18 was the best cutoff point on the CPTCI-S, in that it was indicative of clinically significant appraisals as determined by the presence of PTSD. Based on these results, we concluded that the CPTCI is a useful tool to support the practice of clinicians and that the CPTCI-S has excellent psychometric properties.