This study evaluated and compared the measurement properties of the 13-item FACITFatigue Scale (FACIT-F) and the 9-item Fatigue Severity Scale (FSS) in 118 consecutive Parkinson's disease (PD) patients, using traditional and Rasch measurement methodologies.Both questionnaires exhibited excellent data quality and reliability (coefficient alpha ≥0.9), acceptable rating scale functionality, and discriminated between fatigued and non-fatigued patients. Factor and Rasch analyses provided general support for unidimensionality of both the FACIT-F and FSS, although they do not appear to measure identical aspects of fatigue.No signs of differential item functioning (DIF) were found for the FACIT-F whereas potential age DIF was detected for 2 FSS items. These results support the measurement validity of both questionnaires in PD, although the FACIT-F displayed better measurement precision and modest psychometric advantages over the FSS. Availability of psychometrically sound fatigue measures that are applicable across disorders provides a sound basis for advancing the understanding of this common and distressing complaint. Fatigue can be defined as an overwhelming sense of tiredness, lack of energy, and feeling of exhaustion (1) and is a common complaint in a range of medical conditions, including many neurological disorders (1-3). In a clinical context, fatigue is considered to be a multidimensional concept with physical, emotional, cognitive, and social aspects. This can pose a challenge to measurement. In Parkinson's disease (PD), fatigue has been reported in 40-65% of patients and although many consider it to be one of their most disabling symptoms, it often remains undetected in clinical practice (4)(5)(6)(7)(8). Its cause remains unclear. For example, while some reports suggest an association between fatigue and the underlying parkinsonism (5, 9), others do not (10, 11).One reason for such mixed results may relate to how fatigue has been assessed. Studies have tended to use fatigue scales from generic health status questionnaires or approaches not validated in [10][11][12]. Although generic health status questionnaires have been validated broadly, their subscales are brief and often lack sufficient detail for accurately measuring individuals. For example, the Energy subscale of the Nottingham Health Profile (NHP-EN) (13) has been found to be only a coarse measure of fatigue (14). Other employed instruments have been validated in other patient groups but remain untested in terms of their measurement properties in PD. This is a limitation because traditional psychometric properties are sample dependent. Thus their performance in specific applications is important to consider in the context of accumulated experience with an instrument (15). There is thus a need for fatigue measures with documented reliability and validity in specific patient populations, such as PD, that also allow for comparisons with other patient groups and healthy control populations (3).We therefore sought to identify and validate an availa...