Background: Alemtuzumab efficacy versus subcutaneous interferon-β-1a (SC IFNB-1a) was demonstrated over 2 years in patients with relapsing-remitting multiple sclerosis, with continued efficacy over 7 additional years. Alemtuzumab is included as a recommended treatment for patients with highly active disease (HAD) by the American Academy of Neurology Practice Guidelines, and the label indication in Europe was recently restricted to the treatment of HAD patients. There is currently no consensus definition for HAD, and alemtuzumab efficacy across various HAD definitions has not been explored previously. Objectives: In this post hoc analysis, we assess the efficacy and safety of alemtuzumab in Comparison of Alemtuzumab and Rebif® Efficacy in Multiple Sclerosis (CARE-MS) trial patients who met criteria for at least one of four separate definitions of HAD (one primary and three alternatives). Over 2 years, alemtuzumab-treated HAD patients were compared with SC IFNB-1a-treated HAD patients, with additional 7-year follow-up in patients from the alemtuzumab arm. Methods: Patients in the CARE-MS studies received either alemtuzumab (baseline: 5 days; 12 months later: 3 days) or SC IFNB-1a (3 times weekly). Alemtuzumab-treated patients who enrolled in the extensions could receive additional courses ≥ 12 months apart. Four definitions of HAD were applied to assess alemtuzumab efficacy: the pre-specified primary definition (two or more relapses in the year prior to baseline and at least one gadolinium [Gd]-enhancing lesion at baseline) and three alternative definitions that focused on relapse, magnetic resonance imaging (MRI), or prior treatment response criteria. Efficacy outcomes were annualized relapse rate, change in Expanded Disability Status Scale score, 6-month confirmed disability worsening, 6-month confirmed disability improvement, MRI disease activity, and brain volume change. Adverse events were summarized for HAD patients meeting the primary definition. Results: In the pooled CARE-MS population, 208 alemtuzumab-treated patients met the primary HAD definition. Annualized relapse rate was 0.27 in years 0–2 and 0.16 in years 3–9. Over 9 years, 62% of patients were free of 6-month confirmed disability worsening, 50% had 6-month confirmed disability improvement, and median cumulative change in brain volume was − 2.15%. During year 9, 62% had no evidence of disease activity, and 69% were free of MRI disease activity. Similar efficacy outcomes were observed using an alternative relapse-driven HAD definition. For patients meeting alternative HAD definitions focused on either higher MRI lesion counts or disease activity while on prior therapy, reduced efficacy for some endpoints was seen. Safety was consistent with the overall CARE-MS population through year 9. Conclusions: Over 9 years, alemtuzumab efficacy was maintained in CARE-MS HAD patients based on four HAD definitions. These results support intervention with alemtuzumab in patients with early indicators of HAD, including frequent relapse without high MRI activity. No safety signals were observed over 9 years that were unique to the HAD populations. ClinicalTrials.gov Identifiers: NCT00530348; NCT00548405; NCT00930553; NCT02255656.