Background: In the pivotal phase III 2-year ADVANCE study, subcutaneous peginterferon beta-1a 125 mcg every 2 weeks demonstrated significant improvements in clinical outcomes, including disability endpoints, in patients with relapsing–remitting multiple sclerosis (RRMS). Here, we aim to further evaluate disability data from ADVANCE, and explore associations between confirmed disability progression (CDP), functional status, and health-related quality of life (HRQoL). Methods: In total, 1512 patients were randomized to placebo or peginterferon beta-1a 125 mcg every 2 or 4 weeks. After 1 year, patients on placebo were re-randomized to peginterferon beta-1a every 2 or 4 weeks. CDP was defined as ⩾1.0 point increase from a baseline Expanded Disability Status Scale (EDSS) score ⩾ 1.0, or ⩾1.5-point increase from baseline 0, confirmed 12 or 24 weeks after onset. Results: Peginterferon beta-1a every 2 weeks significantly reduced risk of 12- and 24-week CDP at 1 year compared with placebo (12-week CDP: 6.8% versus 10.5%, p = 0.038; 24-week CDP: 4% versus 8.4%, p = 0.0069, peginterferon beta-1a every 2 weeks versus placebo, respectively). Benefits were maintained over 2 years (11.2% and 7.7%, peginterferon beta-1a every 2 weeks in 12- and 24-week CDP, respectively). Approximately 90% of patients with 24-week CDP had simultaneous worsening by ⩾1 point in at least one functional system score, most commonly pyramidal. Displaying a 24-week CDP was associated with worse scores on the Multiple Sclerosis Functional Composite (MSFC) scale and several HRQoL instruments; the impact of CDP was attenuated by treatment with peginterferon beta-1a every 2 weeks. Conclusions: Peginterferon beta-1a has the potential to prevent/delay worsening of disability in patients with relapsing–remitting multiple sclerosis. Furthermore, improved benefits in disability status with peginterferon beta-1a were also associated with improved functional status and HRQoL [ ClinicalTrials.gov identifier: NCT00906399].