Peripheral blood (PB) and cerebrospinal fluid (CSF) lymphocyte subpopulations, defined by various T-cell specific monoclonal antibodies and flow cytometry, were analysed in 44 relapsing remitting multiple sclerosis (RRMS) patients (including 21 subjects in the acute phase and 23 in the stable phase), 40 chronic-progressive multiple sclerosis (CPMS) patients, and 24 patients with other neurological diseases (OND), in order to verify the presence of any abnormality in the lymphocyte subset pattern. A significant increase in the total number of T-lymphocytes and the CD4+ subpopulation was found in the PB of the MS patients in comparison with the OND group. Moreover, a not statistically significant increase in CD4+ cells was observed in the CSF of MS patients. A statistically significant increase was also found in the CD4+ Leu 8+ (suppressor inducer) cells in the CSF of all of the MS groups. Finally, the CD8+ (suppressor/cytotoxic) cell levels, were significantly lower in the CSF of CPMS and stable RMS patients than in the CSF of the OND patients. As a whole, our data suggest that the immunosuppressive deficit that seems to be a constant finding in MS is not due to a decrease in suppressor inducer cell levels, as previously suggested, but may be caused by a missed or altered signal from the suppressor inducer to CD8+ suppressor cells.