Introduction The aim of this prospective, observational, single centre study was to evaluate the effects of Natalizumab on cognition and disability measures in rapidly evolving relapsing remitting multiple sclerosis (REMS) patients treated for up to six years.

Method We analysed data for 37 patients (26F) treated with Natalizumab for up to 6 years. Data recorded included 3-monthly assessments of expanded disability severity status (EDSS), 10 metre timed walk (10MTW) and Addenbrooke's cognitive examination (ACE-R).

Results Disability (EDSS (SD)) improved from baseline (4.2 (1.8)) and at 3 months (−0.49), was maximal (−1.3) at 3 years, and sustained to 6 years (−1.0). Sustained disability improvement/progression was seen in 37% and 13% respectively.

Cognition (mean ACE-R 88.3) improved at 3 months (mean change 2.2 points) and was sustained (7.8 at 3 years) to 6 years (7.0). The proportion with a sustained improvement in cognition (>4 on ACE-R) was 22% at 3 months and 59% overall.

A clinically significant improvement (>20%) in the 10MTW was noted in 13.9% at 3 months and in 27% overall.

Conclusion The results demonstrate that Natalizumab may improve disability, walking and cognition in a significant proportion of REMS. Improvement was noted within 3 months and maintained throughout treatment.