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Azathioprine is an immunosuppressive agent that reduces relapse rates in patients with multiple sclerosis (MS), but its efficacy in suppressing new brain lesions has never been evaluated.To evaluate the efficacy of azathioprine therapy on new brain lesion suppression in MS.Open-label treatment vs baseline study.Outpatient MS clinical center at a university hospital.Fourteen patients with relapsing-remitting MS of short duration and at least 3 gadolinium-enhancing (Gd+) brain lesions observed within 6 months before treatment.Azathioprine, up to 3 mg/kg daily, individually adjusted according to blood lymphocyte number and the occurrence of adverse events.Brain Gd+ lesions evaluated by monthly magnetic resonance imaging for 6 months before and 6 months during treatment and new T2 lesions evaluated during the same periods and after an additional 6 months.The treatment reduced to 0 the median Gd+ lesion number and volume per magnetic resonance image (P.001 for both), resulting in a Gd+ lesion number reduction of 50% or more in 12 of 14 patients (P.01). An equivalent reduction in the new T2 lesion number was observed (P.02); this activity also persisted during the additional treatment period evaluated using this outcome measure (P.01). The median azathioprine dose administered (2.6-2.8 mg/kg daily) reduced the mean blood lymphocyte count to 57% of the baseline value. Adverse events were transient or reversible with dose adjustment.This study indicates for the first time that azathioprine, administered at lymphocyte-suppressing doses, is effective in reducing MS new brain inflammatory lesions and is well tolerated.