Recommending vitamin D3 supplements in vitamin D insufficient, relapsing-remitting MS patients is associated with increased 25-hydroxyvitamin D (25(OH)D) levels. Increased 25(OH)D levels in these patients are associated with decreases in the annualized relapse rate (ARR). No association was found between changes in 25(OH)D and changes in gene expression of molecules representing the immune system. This reflects how complex vitamin D research is.