BACKGROUND: Adolescents and young adult (AYA) cancer survivors may have unique physical, psychological and social needs due to their cancer occurring at a critical phase of development. The aim of this study was to develop a psychometrically rigorous measure of unmet need to capture the specific needs of this group. METHODS: Items were developed following a comprehensive literature review, focus groups with AYAs, and feedback from health care providers, researchers and other professionals. The measure was pilot tested with 32 AYA cancer survivors recruited through a state-based cancer registry to establish face and content validity. A main sample of 139 AYA cancer patients and survivors were recruited through seven treatment centres and invited to complete the questionnaire. To establish test-retest reliability, a sub-sample of 34 participants completed the measure a second time. Exploratory factor analysis was performed and the measure was assessed for internal consistency, discriminative validity, potential responsiveness and acceptability. RESULTS: The Cancer Needs Questionnaire – Young People (CNQ-YP) has established face and content validity, and acceptability. The final measure has 70 items and six factors: Treatment Environment and Care (33 items); Feelings and Relationships (14 items); Daily Life (12 items); Information and Activities (5 items); Education (3 items); and Work (3 items). All domains achieved Cronbach’s alpha values greater than 0.80. Item-to-item test-retest reliability was also high, with all but four items reaching weighted kappa values above 0.60. CONCLUSIONS: The CNQ-YP is the first multi-dimensional measure of unmet need which has been developed specifically for AYA cancer patients and survivors. The measure displays a strong factor structure, and excellent internal consistency and test-retest reliability. However, the small sample size has implications for the reliability of the statistical analyses undertaken, particularly the exploratory factor analysis. Future studies with a larger sample are recommended to confirm the factor structure of the measure. Longitudinal studies to establish responsiveness and predictive validity should also be undertaken.