OBJECTIVES: Quality of life (QoL) is an important measure in multiple sclerosis (MS), complementing clinical assessments such as the Expanded Disability Status Scale (EDSS). The MS International QoL (MusiQoL) questionnaire is a validated, MS-specific instrument. This multicentre, observational study assessed responsiveness of MusiQoL versus the Short Form-36 version 2 (SF36v2) to changes in EDSS score over 24 months in subjects with MS (≥18 years; EDSS ≤7.0; treated/untreated). METHODS: MusiQoL, SF36v2 and EDSS scores were recorded at baseline (BL) and 6-monthly intervals to month 24 (M24). Primary endpoint: change in MusiQoL index score and effect size (ES) to M24. Secondary endpoints included change in MusiQoL and SF36v2 scores and ES (BL–6-monthly assessments).  RESULTS: Six hundred subjects enrolled in 12 countries; 452 had evaluable BL and M24 EDSS and MusiQoL index data. BL mean (SD) EDSS score was 2.9 (1.9), mean (SD) MusiQoL index score was 68.5 (14.3), and mean (SD) MusiQoL subscale scores ranged from 59.8 (25.0) to 85.5 (18.2). EDSS score worsened by M24 in 89 subjects (19.7% vs expected 30%); mean (SD) change in MusiQoL index score was 0.30 (12.3) in non-worsened (ES: 0.02) and –2.3 (11.6) in worsened (ES: –0.17) subjects. At M24, larger (mean [SD]; ES) changes were seen in MusiQoL Relationship with Healthcare System (–6.0 [16.9]; –0.40) and Sentimental and Sexual Life (–6.4 [26.2]; –0.22) subscale scores for worsened subjects; in Psychological Well-Being (+4.8 [21.6]; 0.20) subscale score for non-worsened subjects; and in the SF36v2 physical component (–2.5 [6.7]; –0.24), physical functioning (–3.3 [7.6]; –0.28), bodily pain (–3.1 [10.3]; –0.28) and emotional (–2.8 [14.1]; –0.22) subscale scores for worsened subjects.  CONCLUSIONS: MusiQoL index score detected poorer QoL in subjects with worsening EDSS scores. Most MusiQoL scores decreased over 24 months in subjects with more severe disability, indicating poorer QoL and confirming the utility of the MS-specific MusiQoL questionnaire in rating QoL.