Purpose: Many youth and adults with Cerebral Palsy (CP) experience high levels of fatigue. This study aimed to compare three fatigue self-report questionnaires to guide clinicians. Method: Thirty youth and adults (age range 17–64) with CP were assessed with Danish versions of the Fatigue Impact and Severity Self-Assessment questionnaire, the Modified Mental Fatigue Scale, and the Multidimensional Fatigue Inventory. Psychometric properties were investigated. Rank order and classification models were compared across questionnaires. Results: The Reduced Motivation and Physical Fatigue subscales of the Multidimensional Fatigue Inventory showed inadequate internal consistency. Participants were frequently ranked differently with the questionnaires. There were issues related to the conceptualization of physical fatigue. Conclusion: The choice of assessment tool should be based on assessment purpose as the questionnaires assess different aspects of fatigue severity, impact, and management. Also, test selection can have important implications on the conclusions that are made about fatigue type and severity.