T-lymphocytes in the cerebrospinal fluid of patients with multiple sclerosis are predominantly CD4+ (inducer) as opposed to CD8+ (suppressor/cytotoxic) T cells. The CD4+ lymphocytes can be subdivided into populations that express high densities of the CDw29 (4B4) determinant and have helper inducer function or express high densities of CD45R (2H4) determinant and have suppressor inducer function. In the present study, we characterized the nature of these CD4+ T cells in the cerebrospinal fluid of patients with multiple sclerosis by performing flow cytometric analysis on paired samples of blood and cerebrospinal fluid. There were significantly lower percentages of CD4+ 2H4+ T cells in the cerebrospinal fluid than in the peripheral blood (p = 0.001, paired t test). In contrast, there were increased percentages of helper inducer (CD4+ 4B4+) T cells in the cerebrospinal fluid (p = 0.001, paired t test), compared with the peripheral blood. Analysis of subjects with other inflammatory disorders of the central nervous system did not show significant decreases in CD4+ 2H4+ T cells in cerebrospinal fluid, though in some, decreases were also observed. These results indicate that the CD4+ T cells in the cerebrospinal fluid of patients with multiple sclerosis are predominantly helper inducer, as opposed to suppressor inducer T cells, and that the relative decrease of suppressor inducer cells in the peripheral blood of multiple sclerosis patients is not due to their migration to the cerebrospinal fluid. Furthermore, the increased numbers of helper inducer cells in the cerebrospinal fluid may contribute to local autoimmune processes in the central nervous system compartment of multiple sclerosis patients.