Pegylation of pharmacological substances was developed in the 1970s as a way of improving their efficacy and elimination and hence reducing the dosage frequency. A prominent example is pegylation of IFNα, which revolutionized the treatment of virus hepatitis in the late 1990s. Efforts have now succeeded in producing a pegylated interferon beta (PEG-IFN-β1a) to treat multiple sclerosis (MS) and the efficacy and safety have been investigated in a phase III trial called the ADVANCE study. The 1-year results of this randomized, double blind, multicenter, placebo-controlled study in more than 1500 MS patients show that administration of subcutaneous PEG-IFN-β1a significantly reduces the annual relapse rate and disability progression. The safety and tolerability profile of PEG-IFN-β1a was found to be similar to that of conventional IFN-β drugs. The most common adverse events were flu-like symptoms and redness at the injection site. The results of this study underscore that PEG-IFN-β1a is an interesting new therapeutic option in the treatment of relapsing-remitting MS that combines highly effective interferon with the established tolerability and safety profile of IFN-β at a reduced dosage frequency.