Multiple sclerosis (MS) is an inflammatory disease of the CNS white matter associated with T cell infiltrates and alterations of immune functions that can be measured in the peripheral immune system. TIM-3 has been identified as a central regulator of IFN-γ-secreting type 1 Th (Th1) cells and immune tolerance. In this study, using a newly generated mAb against human TIM-3, we examined TIM-3 function on ex vivo CD4+ T cells isolated from the circulation of healthy subjects and patients with MS. Blocking TIM-3 during T cell stimulation significantly enhanced IFN-γ secretion in control subjects but had no effect in untreated patients with MS, demonstrating a defect in TIM-3 immunoregulation. Treatment with glatiramer acetate or IFN-β reversed this functional defect. Reduced levels and altered kinetics of T cell TIM-3 expression, which was restored in treated patients, is one mechanism that can explain the loss of TIM-3 regulation of T cell function in untreated patients with MS. These data provide functional, mechanistic data for dysregulated TIM-3 immunoregulation in a human autoimmune disease and suggest that approved therapies for the treatment of MS may function in part by restoring TIM-3 immunoregulation of T cell function.