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Background: Progression in disability as measured by increase in the Expanded Disability Status Scale (EDSS) is commonly used as outcome variable in clinical trials concerning multiple sclerosis (MS). In this study, we addressed the question, whether there is a linear relationship between disability status and health related quality of life (HRQOL) in MS.Methods: 7305 MS patients were sent a questionnaire containing a German version of the "Multiple Sclerosis Quality of Life (MSQOL)-54" and an assessment of self-reported disability status analogous to the EDSS. 3157 patients participated in the study. Patients were allocated to three groups according to disability status.Results: Regarding the physical health composite and the mental health composite as well as most MSQOL-54 subscales, the differences between EDSS 4.5-6.5 and EDSS > or = 7 were clearly smaller than the differences between EDSS < or = 4 and EDSS 4.5-6.5.Conclusion: These results indicate a non-linear relationship between disability status and HRQOL in MS. The EDSS does not seem to be interval scaled as is commonly assumed. Consequently, absolute increase in EDSS does not seem to be a suitable outcome variable in MS studies. [ABSTRACT FROM AUTHOR]