Background Teriflunomide is a disease modifying treatment (DMT) approved for relapsing-remitting multiple sclerosis (RRMS) in adults and children. It reduces lymphocyte proliferation by inhibiting the mitochondrial enzyme dihydroorotate dehydrogenase (DHODH) and thereby the pyrimidine synthesis. Although most DMTs in multiple sclerosis (MS) modulate or inhibit the immune system in the periphery, the efficacy may improve if the agent also targets immune activity within the central nervous system (CNS), acts as a neuro-protective and enhances neuro-regeneration. The objective of this study was to determine the passage of teriflunomide over the blood-cerebrospinal fluid barrier (BCSFB). Methods Plasma and cerebrospinal fluid (CSF) teriflunomide concentrations were determined at steady state in 12 patients with RRMS, treated with oral teriflunomide 14 mg once daily. Included patients were all clinically stable without relapse or disability worsening within 6 months prior from baseline and were on no other immune modulating or immunosuppressive drugs. ResultsThe mean teriflunomide concentrations in plasma and CSF were 38775 (SEM ± 7256) ng/mL and 68 (SEM ± 15) ng/mL, respectively. The passage over the BCSFB was 0.17 % (SEM ± 0.01). While no correlation was found between the function of the BCSFB assessed with the albumin ratio and the CSF teriflunomide concentration, the CSF and plasma teriflunomide concentrations were highly correlated (r s = 0.90, < 0.0001). Conclusions Further studies are warranted to determine if the obtained CSF teriflunomide concentration reflects that in the CNS and is able to influence inflammatory and degenerative processes within the CNS.