In several studies, Multiple Sclerosis (MS) incidence and disease activity has been related with vitamin D status. We observed that RRMS patients who remained relapse free before blood collection had a better vitamin D status than patients who experienced relapses (Smolders et al. Mult Scler 2008;17:1220-1224). Since vitamin D3 is a potent promotor of T cell regulation in vitro (Smolders et al. J Neuroimmunol 2008;194:7-17), we hypothesised that a promotion of Treg function in MS patients might underlie its association with MS disease activity. In a cohort of RRMS patients, we observed a positive correlation of Treg function with vitamin D status (Smolders et al. PLoS ONE 2009;4:e6635). Furthermore, vitamin D status correlated positively with a Th1/Th2-balance which was more directed towards Th2. In the present study, we will assess whether treatment of RRMS patients with vitamin D3 promotes T cell regulation.