Two prior double-blind, placebo-controlled, randomized trials demonstrated that glatiramer acetate (GA) reduces relapse rates in patients with relapsing remitting multiple sclerosis (RRMS). This study was designed to determine the effect, onset, and durability of any effect of GA on disease activity monitored with magnetic resonance imaging (MRI) in patients with RRMS. Two hundred thirty-nine eligible patients were randomized to receive either 20 mg GA (n = 119) or placebo (n = 120) by daily subcutaneous injection. Eligibility required one or more relapses in the 2 years before entry and at least one enhancing lesion on a screening MRI. The study was a randomized, double-blind, placebo-controlled phase during which all patients studied underwent monthly MRI scans and clinical assessments over 9 months. The primary outcome measure was the total number of enhancing lesions on T1-weighted images. Secondary outcome measures included the proportion of patients with enhancing lesions, the number of new enhancing lesions and change in their volume; the number of new lesions detected on T2-weighted images and change in their volume, and the change in volume of hypointense lesions seen on unenhanced T1-weighted images. Clinical measures of disease activity were also evaluated. The active treatment and placebo groups were comparable at entry for all demographic, clinical, and MRI variables. Treatment with GA showed a significant reduction in the total number of enhancing lesions compared with placebo (-10.8, 95% confidence interval -18.0 to -3.7; p = 0.003). Consistent differences favoring treatment with GA were seen for almost all secondary end points examined: number of new enhancing lesions (p < 0.003), monthly change in the volume of enhancing lesions (p = 0.01), and change in volume (p = 0.006) and number of new lesions seen on T2-weighted images (p < 0.003). The relapse rate was also significantly reduced by 33% for GA-treated patients (p = 0.012). All effects increased over time. Glatiramer acetate significantly reduced MRI-measured disease activity and burden.