Mesenchymal stem cells (MSCs) can abrogate the animal model of multiple sclerosis, experimental autoimmune encephalomyelitis (EAE), but whether this therapeutic effect occurs entirely through systemic immune modulation and whether CNS infiltration occurs after peripheral delivery are uncertain. We studied the clinical and neuropathologic effects of intravenously administered human MSCs (hMSCs) in C57BL/6 mice with EAE. Human MSCs significantly reduced the clinical disease severity, particularly in later disease. Large numbers of hMSCs migrated into gray and white matter at all levels of the spinal cord in both naive mice and mice with EAE. In the latter, hMSCs accumulated over time in demyelinated areas. There were 2 distinct morphological appearances of the hMSCs in the tissue, that is, rounded and less numerous process-bearing forms; very few expressed neural markers. The number of spinal cord white matter lesions and areas of white matter demyelination were reduced after hMSC treatment compared with control treatment. These findings show that central nervous system infiltration occurs after peripheral delivery of hMSCs, that they accumulate where there is myelin damage, and that they are associated with a reduced extent of demyelination. These data support a potential role for hMSCs in autologous cell therapy in multiple sclerosis.