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Background The main goal of treatment in relapsing-remitting multiple sclerosis (RRMS) is to reduce the occurrence of relapses. However, little is known about the natural history of relapse recovery.Methods We accessed data from DECIDE (n=1841), a phase 3 trial. We investigated factors associated with time to relapse recovery (defined as a return of the Expanded Disability Status Scale (EDSS) score to the pre-relapse level or lower), relapse severity (0.5, 1.0, or >1.0 EDSS score change) and the new concept of 'acute clinical events with stable MRI' (ACES). Variables used were age, sex, disease duration, treatment arm, pre-relapse EDSS, corticosteroid use, number of relapses prior to study enrolment, MRI activity, relapse severity and affected functional system (FS).Results We included 430 first relapses, of which 405 (94.2%) recovered during follow-up, 400 (93%) by 1 year (median time to recovery of 71 days, 95% CI 66 to 75 days). More severe relapses and relapses involving the bowel and bladder FS took a longer time to recover. Corticosteroids hastened the recovery of relapses but did not influence eventual relapse recovery. ACES occurred in 38% of relapses and was more frequent in older people and participants treated with daclizumab.Conclusions Most relapses (94.2%) recover, but the process of recovery can take up to 1 year and depends mostly on relapse severity. Our findings challenge the concept of 3-month and 6-month confirmed disability progression as reliable markers of permanent disability in RRMS trials. ACES occurs frequently and is associated with age.