Purpose: Cancer is the leading cause of nonaccidental deaths among adolescents and young adults (AYAs). In Denmark, there are substantial gaps in knowledge concerning how AYAs with cancer perceive their diagnostic and therapeutic trajectory and report health-related outcomes. The aim of this study is to describe the development of a questionnaire targeting AYAs with cancer aiming to evaluate treatment and survivorship from the perspective of the patients. Methods: Identification of themes and development of items included in the questionnaire were based on a synthesis of literature and qualitative interviews with AYAs in an iterative process involving both a professional advisory panel and a youth panel. During the development process, items were validated through cognitive interviews. Results: The final questionnaire contained 151 closed- and open-ended items divided into 6 sections regarding: (1) "Time before treatment," (2) "Being told about your illness," (3) "Being a young patient," (4) "Your treatment," (5) "Receiving help living with and after Cancer," and (6) "How are you feeling today?." One hundred one items were specifically developed for this study, while 50 were standardized validated indexes. The questionnaire combined different types of items such as needs, preferences, experiences, and patient-reported outcomes. Conclusion: This is one of few developed questionnaires aiming to evaluate the perspective of AYAs with cancer through their whole cancer trajectory. Results from the questionnaire survey are intended for quality improvements and research in AYA cancer care. The study highlights the importance of an extensive patient involvement in all steps of a questionnaire development process.