The primary objective of this study is to evaluate the long-term safety and tolerability of peginterferon beta-1a (BIIB017) in participants originally treated in Study 105MS301 (NCT00906399) who continue peginterferon beta-1a treatment. The secondary objective of this study is to describe long-term multiple sclerosis (MS) outcomes in participants originally treated in Study 105MS301 (NCT00906399) who continue peginterferon beta-1a treatment.