A combination of genetic and environmental factors probably plays a role in determining an increased susceptibility to multiple sclerosis (MS). Among these factors, vitamin D and A metabolites are likely to play a role given their immunomodulatory properties. Decreased serum vitamin D levels have been associated with clinical and MRI activity of MS. Løken-Amsrud et al. evaluated the association of retinol concentration with clinical and MRI measures of disease activity in MS patients over a 2-year period. Serum retinol levels correlated with MRI metrics of disease activity, but not with clinical findings. Following IFN-β-1a treatment, the association with MRI metrics was lost. These results support a role of vitamin A metabolites in influencing disease activity in MS.