Summary Background Subcutaneous pegylated interferon (peginterferon) beta-1a is being developed for treatment of relapsing multiple sclerosis, with less frequent dosing than currently available first-line injectable treatments. We assessed the safety and efficacy of peginterferon beta-1a after 48 weeks of treatment in the placebo-controlled phase of the ADVANCE trial, a study of patients with relapsing-remitting multiple sclerosis. Methods We did this 2-year, double-blind, parallel group, phase 3 study, with a placebo-controlled design for the first 48 weeks, at 183 sites in 26 countries. Patients with relapsing-remitting multiple sclerosis (age 18–65 years, with Expanded Disability Status Scale score ≤5) were randomly assigned (1:1:1) via an interactive voice response or web system, and stratified by site, to placebo or subcutaneous peginterferon beta-1a 125 μg once every 2 weeks or every 4 weeks. The primary endpoint was annualised relapse rate at 48 weeks. This trial is registered with ClinicalTrials.gov , number NCT00906399. Findings We screened 1936 patients and enrolled 1516, of whom 1512 were randomly assigned (500 to placebo, 512 to peginterferon every 2 weeks, 500 to peginterferon every 4 weeks); 1332 (88%) patients completed 48 weeks of treatment. Adjusted annualised relapse rates were 0·397 (95% CI 0·328–0·481) in the placebo group versus 0·256 (0·206–0·318) in the every 2 weeks group and 0·288 (0·234–0·355) in the every 4 weeks group (rate ratio for every 2 weeks group 0·644, 95% CI 0·500–0·831, p=0·0007; rate ratio for the every 4 weeks group 0·725, 95% CI 0·565–0·930, p=0·0114). 417 (83%) patients taking placebo, 481 (94%) patients taking peginterferon every 2 weeks, and 472 (94%) patients taking peginterferon every 4 weeks reported adverse events including relapses. The most common adverse events associated with peginterferon beta-1a were injection site reactions, influenza-like symptoms, pyrexia, and headache. 76 (15%) patients taking placebo, 55 (11%) patients taking study drug every 2 weeks, and 71 (14%) patients taking study drug every 4 weeks reported serious adverse events; relapse, pneumonia, and urinary tract infection were the most common. Interpretation After 48 weeks, peginterferon beta-1a significantly reduced relapse rate compared with placebo. The drug might be an effective treatment for relapsing-remitting multiple sclerosis with less frequent administration than available treatments. Funding Biogen Idec.