Interferon-beta is a biological treatment widely used in multiple sclerosis (MS). However, not every patient responds equally well to this therapy. In this study, our aim was to evaluate the influence of a dinucleotide microsatellite located in the first intron of the interferon-gamma gene on relapse eradication in a group of interferon-beta-treated patients. Our results show a very different allelic distribution when patients with relapses were compared with relapse-free patients.