BACKGROUND: Adverse event (AE) reporting in oncology trials is required, but current practice does not directly integrate the child's voice. The Pediatric Patient-Reported Outcomes version of the Common Terminology Criteria for Adverse Events (PRO-CTCAE) is being developed to assess symptomatic AEs via child/adolescent self-report or proxy-report. This qualitative study evaluates the child's/adolescent's understanding and ability to provide valid responses to the PRO-CTCAE to inform questionnaire refinements and confirm content validity. PROCEDURE: From seven pediatric research hospitals, children/adolescents ages 7-15 years who were diagnosed with cancer and receiving treatment were eligible, along with their parent-proxies. The Pediatric PRO-CTCAE includes 130 questions that assess 62 symptomatic AEs capturing symptom frequency, severity, interference, or presence. Cognitive interviews with retrospective probing were completed with children in the age groups of 7-8, 9-12, and 13-15 years. The children/adolescents and proxies were interviewed independently. RESULTS: Two rounds of interviews involved 81 children and adolescents and 74 parent-proxies. Fifteen of the 62 AE terms were revised after Round 1, including refinements to the questions assessing symptom severity. Most participants rated the PRO-CTCAE AE items as "very easy" or "somewhat easy" and were able to read, understand, and provide valid responses to questions. A few AE items assessing rare events were challenging to understand. CONCLUSIONS: The Pediatric and Proxy PRO-CTCAE performed well among children and adolescents and their proxies, supporting its content validity. Data from PRO-CTCAE may improve symptomatic AE reporting in clinical trials and enhance the quality of care that children receive.