The aetiology of subacute sclerosing panencephalitis (SSPE) remains to be fully elucidated, although it follows infection with a hypermutant defective M-protein measles virus. This study analysed peripheral blood lymphocyte subgroups to determine their role in the pathophysiology of SSPE. It included 22 children with SSPE aged 2 - 15 years (patient group) and 22 age- and gender-matched healthy children (control group). In children < 6 years old, there were no statistically significant differences between the two groups in the proportions of lymphocytes expressing the surface markers CD3, CD8, CD19 or CD16/56, or in CD4/CD8 ratio. The proportion of CD4(+) lymphocytes in SSPE patients < 6 years of age was significantly lower compared with the control group. In children >= 6 years old, there were no significant differences in the lymphocyte subgroups. In conclusion, these findings suggest that a low CD4(+) lymphocyte count might be responsible for SSPE in younger children.