An improved understanding of the immunopathogenesis of multiple sclerosis (MS) has led to therapeutic attempts using immunoregulatory agents in patients with MS, including interferon beta. IFN-beta 1a has been tested in relapsing-remitting MS and its effectiveness in reducing exacerbation rate and in slowing sustained worsening of disability after 2 years has been shown. Magnetic resonance imaging results supported the clinical findings, showing a significant reduction in Gd-enhancing lesion frequency and new lesion formation on T2-weighted images. This type of interferon is well tolerated and most adverse events are mild and transient. Further trials are in progress to amplify and clarify the observed benefits of IFN-beta 1a.