Recently published studies in multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE) have demonstrated an association between the development of demyelinating plaques and the accumulation of Th17 cells in the central nervous system and periphery. However, a causal relationship has been difficult to establish. In fact, in reports published thus far, interleukin (IL)-17A deficiency or neutralization in vivo attenuates, but does not completely abrogate, EAE. There is growing evidence that clinically similar forms of autoimmune demyelinating disease can be driven by myelin-specific T cells of distinct lineages with different degrees of dependence on IL-17A production to achieve their pathological effects. While such observations cast doubts about the potential therapeutic efficacy of Th17 blocking agents in MS, the collective data suggest that IL-17A expression in peripheral blood mononuclear cells could serve as a surrogate biomarker of neuroinflammation and plaque formation and be a useful outcome measure for future clinical trials.