Background The 21-item Modified Fatigue Impact Scale (MFIS) has been recommended as an outcome measure for use in multiple sclerosis and is commonly used to generate an overall score of fatigue.

Objective To test if the MFIS total score is valid by application of the Rasch measurement model.

Method The MFIS was sent by post to patients with clinically definite multiple sclerosis in two centres in the UK. Data were fitted to the Rasch model.

Results Analysis was based on 415 records (55% response). The 21-item scale did not fit the Rasch model mainly because of multidimensionality. The scale was found to contain a “physical” dimension and a “cognitive” dimension, consistent with the original subscale structure. Valid physical and cognitive subscales were derived after deletion of some items.

Conclusion The MFIS cannot be used to generate a single overall score of fatigue. The conceptual interaction between the two dimensions remains unclear, which poses problems when interpreting change scores in these individual scales. Studies in which a global MFIS score was used as either an outcome measure or selection tool may need to be re-evaluated.