BACKGROUND: Before the introduction of the immunomodulatory therapies for multiple sclerosis (MS), treatment options for MS consisted of symptomatic management (physical therapy and pharmacological treatment for symptom management). Symptomatic management for MS has been supplemented in the past decade by 2 new classes of immunomodulatory therapies that have been approved as first-line treatments for relapsing-remitting multiple sclerosis (RRMS): subcutaneous glatiramer acetate (SC GA) and 3 b-interferons: intramuscular interferon b-1a (IM IFNb-1a), SC IFNb-1a, and SC IFNb-1b. OBJECTIVE: To estimate the economic outcomes of 5 treatment strategies: symptom management alone, symptom management combined with SC GA, IM IFNb1-a, SC IFNb1-a, or SC IFNb1-b in patients diagnosed with RRMS. METHODS: A literature-based Markov model was developed to assess the cost-effectiveness of 5 treatment strategies for managing a hypothetical cohort of patients diagnosed with RRMS in the United States - 4 immunomodulatory drug therapies and symptom management alone. Health states were based on the Kurtzke Expanded Disability Status Scale (EDSS), a widely accepted scale for assessing RRMS (higher EDSS scores = increased disease severity). Baseline relapse and disease progression transition probabilities for symptom management were obtained from natural history studies. Treatment effects of the immunomodulatory therapies were estimated by applying a percentage reduction to the symptom management transition probabilities for relapse (27% reduction) and disease progression (30% reduction). Transition probabilities were subsequently adjusted to account for (1) the effects of neutralizing antibodies, specifically on relapse rates by assuming no additional therapy benefits after the second year of continuous therapy, and (2) treatment discontinuation. Therapy-specific data were obtained from clinical trials and long-term follow-up observational studies. Transitions among health states occurred in 1-month cycles for the lifetime of a patient. Costs (2005 US$) and outcomes were discounted at 3% annually. RESULTS: The incremental cost per quality-adjusted life-year for the 4 immunomodulatory therapies is $258,465, $303,968, $416,301, and $310,691 for SC GA, IM IFNb-1a, SC IFNb-1a, and SC IFNb-1b, respectively, compared with symptom management alone. Sensitivity analyses demonstrated that results were sensitive to changes in utilities, disease progression rates, time horizon, and immunomodulatory therapy cost. CONCLUSIONS: The pharmacoeconomic model determined that SC GA was the best strategy of the 4 immunomodulatory therapies used to manage MS and resulted in better outcomes than symptom management alone. Sensitivity analyses indicated that the model was sensitive to changes in a number of key parameters, and thus changes in these key parameters would likely influence the estimated cost-effectiveness results. Head-to-head randomized clinical trials comparing the immunomodulatory therapies for the treatment of MS are necessary to validate the projections from the pharmacoeconomic analyses, particularly since the results available today from the clinical trials do not account adequately for treatment dropouts.