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Purpose: The Multiple Sclerosis Quality of Life-54 (MSQOL-54) is a specific multiple sclerosis (MS) health-related quality of life inventory consisting of 52 items organized into 12 subscales plus two single items. No study was found in literature assessing its measurement invariance across language versions. We investigated whether MSQOL-54 items provide unbiased measurements of underlying constructs across Italian and English versions., Methods: Three constrained levels of measurement invariance were evaluated: configural invariance where equivalent numbers of factors/factor patterns were required; metric invariance where equivalent factor loadings were required; and scalar invariance where equivalent item intercepts between groups were required. Comparative fit index (CFI), root mean square error of approximation (RMSEA), and standardized root mean square residual (SRMR) fit indices and their changes between nested models were used to assess tenability of invariance constraints., Results: Overall, the dataset included 3669 MS patients: 1605 (44%) Italian, mean age 41 years, 62% women, 69% with mild level of disability; 2064 (56%) English-speaking (840 [41%] from North America, 797 [39%] from Australasia, 427 [20%] from UK and Ireland), mean age 46 years, 83% women, 54% with mild level of disability. The configural invariance model showed acceptable fit (RMSEA = 0.052, CFI = 0.904, SRMR = 0.046); imposing loadings and intercepts equality constraints produced negligible worsening of fit (ΔRMSEA < 0.001, ΔCFI = - 0.002, ΔSRMR = 0.002 for metric invariance; ΔRMSEA = 0.003, ΔCFI = - 0.013, ΔSRMR = 0.003 for scalar invariance)., Conclusions: These findings support measurement invariance of the MSQOL-54 across the two language versions, suggesting that the questionnaire has the same meaning and the same measurement paramaters in the Italian and English versions.