BACKGROUND Treatments affect both relapse-related disability and short-term disability change, but measurements of their impact on long-term outcomes remain a challenge. OBJECTIVE To ascertain the contribution of relapse-associated disability to overall disability in relapse-onset multiple sclerosis (RMS) using long-term data collected in our clinic. METHODS Retrospective study of a cohort of newly diagnosed patients with RMS, (n = 176) was undertaken, measuring all confirmed changes in disability up to 15 years after onset. Worsening was assessed yearly and in 5-year epochs and was attributed to either relapse (RW) or slow progression (PW). RESULTS At data lock, 139/176 (81%) of patients were still actively followed, with Expanded Disability Status Scale (EDSS) available for 10 years post-onset in 145/176 (82%) patients and 15 years post-onset EDSS in 83 patients (mean follow-up entire group 12.7 years post-onset). RW accounted for a large amount of worsening seen in the first 15 years of RMS. RW was less frequent over time, but accounted for most EDSS changes in the first decade of MS (167/267, 63% of EDSS changes), and remained important even in years 11-15 (17/50, 34% of EDSS changes). Median change in disability due to RW vs PW was similar over the entire 15 years. CONCLUSIONS Worsening of treated MS was associated with relapses in many RMS patients throughout the first 15 years after onset, suggesting an opportunity for long-term benefit through relapse reduction.