Participants in the North American Research Committee on Multiple Sclerosis (NARCOMS) Registry report disability using Performance Scales (PS), which assess eight domains. We aimed to validate the PS fatigue subscale (PSF) using the Fatigue Severity Scale (FSS) and the Modified Fatigue Impact Scale (MFIS) as external criteria. We measured correlations between the PSF and age, disease duration and PS subscales to assess construct validity. The PSF correlated strongly with the MFIS (r =0.72, P <0.0001) and FSS (r =0.75, P <0.0001). Correlations between the PSF and age, disease duration were low, indicating divergent validity. Correlations between the PSF and spasticity, sensory and mobility PS scales were moderate, indicating convergent validity. The PSF has adequate criterion and construct validity in MS.