Background: 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. Objective: To evaluate the efficacy of azathioprine therapy on new brain lesion suppression in MS. Design: Open-label treatment vs baseline study. Setting: Outpatient MS clinical center at a university hospital. Patients: Fourteen patients with relapsing-remitting MS of short duration and at least 3 gadolinium-enhancing(Gd+) brain lesions observed within 6 months before treatment. Intervention: Azathioprine, up to 3 mg/kg daily, individually adjusted according to blood lymphocyte number and the occurrence of adverse events. Main Outcome Measures: 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. Results: 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. Conclusions: 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. Background: Azathioprine is an immunosuppressive agent that reduces the relapse rates in patients with multiple sclerosis (MS), but its efficacy in suppressing new brain lesions has never been evaluated. Objective: To evaluate the efficacy of azathioprine therapy on new brain lesion suppression in MS. Setting: Outpatient MS clinical center at a university hospital. Patients: Fourteen patients with relapsing-remitting MS of short duration and at least 3 gadolinium-enhancing (Gd +) brain lesions observed within 6 months before Treatment: Intervention: Azathioprine, up to 3 mg / kg daily, individually adjusted according to blood lymphocyte number and the occurrence of adverse events. Main Outcome Measures: Brain Gd + patients 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. Results: The treatment reduced to 0 the media nGd + 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 this activity also persisted during the additional treatment period as evaluated using this outcome measure (P <.01). The median azathioprine dose administered (2.6-2.8 mg / kg daily) reduced the Adverse events were transient or reversible with dose adjustment. Conclusions: 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.