BACKGROUND: In the 2-year CLARITY study, cladribine tablets significantly improved clinical and magnetic resonance imaging (MRI) outcomes (vs placebo) in patients with relapsing-remitting multiple sclerosis (MS). OBJECTIVE: To assess the safety and efficacy of cladribine treatment in a 2-year Extension study. METHODS: In this 2-year Extension study, placebo recipients from CLARITY received cladribine 3.5 mg/kg; cladribine recipients were re-randomized 2:1 to cladribine 3.5 mg/kg or placebo, with blind maintained. RESULTS: A total of 806 patients were assigned to treatment. Adverse event rates were generally similar between groups, but lymphopenia Grade ⩾ 3 rates were higher with cladribine than placebo (Grade 4 lymphopenia occurred infrequently). In patients receiving cladribine 3.5 mg/kg in CLARITY and experiencing lymphopenia Grade ⩾ 3 in the Extension, >90% of those treated with cladribine 3.5 mg/kg and all treated with placebo in the Extension, recovered to Grade 0-1 by study end. Cladribine treatment in CLARITY produced efficacy improvements that were maintained in patients treated with placebo in the Extension; in patients treated with cladribine 3.5 mg/kg in CLARITY, approximately 75% remained relapse-free when given placebo during the Extension. CONCLUSION: Cladribine tablets treatment for 2 years followed by 2 years' placebo treatment produced durable clinical benefits similar to 4 years of cladribine treatment with a low risk of severe lymphopenia or clinical worsening. No clinical improvement in efficacy was apparent following further treatment with cladribine tablets after the initial 2-year treatment period in this trial setting.