Patients relapsing from multiple sclerosis (MS) are treated with high-dose, short-term intravenous injection of glucocorticoid (GC), although its mechanism of action remains only partly understood. We evaluated the ex vivo and in vitro effects of GC on regulatory T cell (Treg) function in 14 relapsing–remitting MS (RR-MS) patients in acute phase and 20 healthy controls (HC). Treg function was enhanced significantly after 5 days of GC treatment. Furthermore, there was a trend towards increasing proportions of CD4+CD25+forkhead box P3+ T cells and interleukin-10 secretion with GC treatment when compared with HC. In conclusion, GC treatment restores the impaired Treg function in patients with RR-MS in its acute phase.