Fatigue is a complex phenomenon that, for those not affected, is hard to understand. To achieve better assessments, caregivers need reliable and valid tools. The aim of this study was to investigate the reliability and validity of the Swedish version of the Fatigue Impact Scale (FIS) among working-aged individuals diagnosed with multiple sclerosis (MS), as well as in a comparative group randomly selected from the general population in the same geographical area. Both individuals with MS ( n =161) and individuals recruited from the general population ( n =194) participated in the study. A questionnaire was used for the data collection. The data were analysed using non-parametric statistical methods. Reliability of FIS was addressed by item-to-item and item-to-total correlations. Concurrent validity was tested for by analysing correlations between the FIS and general questions, and construct validity by investigation of differences in the FIS scores between known groups. The FIS was found to be homogenous, with item-to-total correlation coefficients of 0.42 ≤ r s ≤0.86 ( p [ABSTRACT FROM AUTHOR]