$Objective$: Evaluate 5-year efficacy and safety of alemtuzumab in treatment-naive patients with active relapsing-remitting multiple sclerosis (RRMS).

$Methods$: Alemtuzumab-treated patients in CARE-MS I (NCT00530348) received treatment courses at Months 0 and 12; after the core study, they could enter an extension (NCT00930553) with as-needed alemtuzumab retreatment for relapse or MRI activity. Assessments included annualized relapse rate (ARR), 6-month confirmed disability worsening (CDW; ≥1-point Expanded Disability Status Scale [EDSS] score increase [≥1.5 if baseline EDSS = 0]), 6-month confirmed disability improvement (CDI; ≥1-point EDSS decrease [baseline score ≥2.0]), no evidence of disease activity (NEDA), brain volume loss (BVL), and adverse events (AEs).

$Results$: Most alemtuzumab-treated patients (95.1%) who completed CARE-MS I enrolled in the extension; 68.5% received no additional alemtuzumab treatment. ARR remained low in Years 3, 4, and 5 (0.19, 0.14, 0.15). Over Years 0–5, 79.7% were free of 6-month CDW; 33.4% achieved 6-month CDI. Most patients (61.7%, 60.2%, 62.4%) had annual NEDA in Years 3, 4, and 5. Median yearly BVL improved over Years 2–4, and remained low in Year 5 (Year 1–5: –0.59%, –0.25%, –0.19%, –0.15%, –0.20%). Exposure-adjusted incidence rates of most AEs declined in the extension relative to the core study. Thyroid disorder incidences peaked at Year 3 and subsequently declined.

$Conclusions$: Based on these data, alemtuzumab has the potential to provide durable efficacy through 5 years in the absence of continuous treatment.

$Classification\; of\; Evidence$: This study provides Class III evidence that alemtuzumab durably improves efficacy outcomes and slows BVL in RRMS patients.