Activated T cells are implicated in the pathogenesis of multiple sclerosis (MS), an autoimmune demyelinating disease of the central nervous system. Serial measurements of T cell activation molecules and T cell subpopulations were performed over 12 months in patients with chronic progressive multiple sclerosis and healthy controls, and correlated with clinical indices of disease progression measured by standardized disability scores. Of the markers studied, the activation molecule CD26 appeared to exhibit a more consistent pattern and to be elevated in MS patients; therefore we concentrated our attention on this marker, especially in view of recent evidence of its role in T cell activation. In this small patient group, the elevation of CD26 in the MS patients did not reach statistical significance, when compared to the level in the controls. Interestingly, the percentage of CD26 but not CD25 or HLA-DR correlated with the MS patients' disability scores. However, the clinical significance of this observation as an indication of disease activity in chronic progressive MS remains to be demonstrated in studies of larger patient populations.