In order to clarify the immunological characteristics of multiple sclerosis (MS) and neuromyelitis optica (NMO), we analyzed CD3, CD4, CD8, CD20, CD4+CD25+, CD4+CD29+, and CD8+CD11ahigh cells in peripheral blood from patients with MS (16 stable, 6 active) and NMO (15 stable, 7 active), as well as 9 with NMO spectrum, 6 with clinically isolated syndrome (CIS), and 13 with other neurological diseases using flow cytometry. Significant decreases in the numbers of CD8+ CD11ahigh cells were observed in stable and active MS and CIS. Our findings indicate that CD8+CD11ahigh cells play different roles in MS and NMO, and their presence may be related to the pathogenesis of MS from the early stage. Copyright © 2010 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]