The aims of this study were to test the validity, test-retest reliability, and internal consistency of Turkish version of FIS; the variables affecting FIS score. 71 MS patients and 68 healthy subjects were included to the study. Total FIS score and subscale scores were different statistically between MS patients and healthy volunteers in both first and second FIS applications (p<0.001). These results showed that FIS is validated in divergent direction. BDI score was higher in MS patients than healthy volunteers (p<0.001). There was no statistically significant difference between two study groups for cognitive subscale scores, after the effect of depression was eliminated (p>0.05). To assess the test-retest reliability, the scores of two FIS applications did not differ statistically (cognitive t=1.948 p>0.05, physical t=1.420 p=0.160, social t=1.470 p=0.146, total t=1.990 p=0.05). Intraclass correlation coefficients were 0.89 (99% confidence interval: 0.79-0.94) for cognitive, 0.95 (0.91-0.97) for physical, 0.91 (0.83-0.95) for social, and 0.93 (0.86-0.96) for total FIS scores (p<0.001). EDSS correlated with physical subscores in both applications of FIS. Turkish version of FIS, which is valid and reliable, seems an appropriate tool for the assessment of the effects of fatigue in Turkish MS population.