In 2014 after phase 3 ADVANCE clinical study was finished, a new, pegylated form of interferon beta-1a with less frequent dosing was accepted for treatment in relapsing-remitting multiple sclerosis. One thousand five hundred and twelve patients with relapsing-remitting multiple sclerosis were enrolled to the ADVANCE study from 183 sites in 26 countries (500 to the placebo group, 512 to the 125 μg subcutaneous peginterferon beta-1a every 2 weeks group and 500 to the 125 μg subcutaneous peginterferon beta-1a every 4 weeks group). The investigated groups were similar in terms of age, sex, duration of the disease and disability rated using the Expanded Disability Status Scale. The primary and secondary endpoints were efficacy and safety of 2-year peginterferon beta-1a treatment in patients with relapsing-remitting multiple sclerosis compared to the placebo group, which after 1 year also received peginterferon beta-1a 125 μg every 2 or every 4 weeks. The results from the 2-year ADVANCE study demonstrate efficacy of treatment with peginterferon beta-1a 125 μg administered subcutaneously every 2 weeks compared with placebo: significantly reduced annualized relapse rate (by 37%), the number of new/newly enlarged T2 lesions (by 67%), the risk of relapse (by 39%) and the risk of 12-week sustained disability progression (by 33%). The most common adverse events (94% of patients) associated with peginterferon beta-1a treatment were: injection site reactions, flu-like symptoms, pyrexia and headache. Sixteen percent of patients taking the study drug every 2 weeks and 22% of patients taking the study drug every 4 weeks reported serious adverse events; relapse, pneumonia and urinary tract infections were the most common. Interpretation: the treatment with peginterferon beta-1a with less frequent administration is effective, well tolerated and safe for patients with relapsing-remitting multiple sclerosis.