Disease-modifying drugs are initiated early and continued for years in patients with multiple sclerosis. Long-term tolerability and impact are not known. The objective of this study was to evaluate long-term tolerability of intramuscular interferon beta-1a and effects on disability and quality of life. Patients were evaluated an average of 15 years after randomization into a placebo-controlled, double-blind trial of intramuscular interferon beta-1a for relapsing multiple sclerosis. Patient-reported Expanded Disability Status Scale, the Short Form-36, a visual analog scale of self-care independence, and a living situation questionnaire were administered. Status was ascertained in 79% (136/172) of eligible patients. Analysis focused on 122 living patients. Despite open-label, non-standardized treatment after the 2-year clinical trial, 46% (n= 56) of the patients remained on intramuscular interferon beta-1a. Expanded Disability Status Scale scores were correlated highly with Short Form-36 subcategories and visual analog scale scores. Patients currently using intramuscular interferon beta-1a had a significantly lower mean Expanded Disability Status Scale score (p= 0.011), less progression to Expanded Disability Status Scale milestones, significantly better scores on the physical component of the Short Form-36 (p< 0.0001), and reported better general health and greater independence. We conclude that patients continuing to use intramuscular interferon beta-1a had less disability and better quality of life compared with patients not currently using intramuscular interferon beta-1a 15 years after randomization into a clinical trial.