OBJECTIVES: Estimate the cost-utility of interferon beta 1B (IFNB-1b) in patients with a clinically isolated syndrome (CIS) suggestive of multiple sclerosis (MS) METHODS: A Markov model was developed using the epidemiology and treatment of CIS and MS. The model simulates outcomes over time horizons ranging from 2 years to lifetime. A hypothetical cohort of 1,000 patients with incident CIS, with initial health states defined by Kurtzke’s Expanded Disability Symptom Scale (EDSS), was specified. The cohort was assumed alternatively to be treated with IFNB-1b (250mg eod) following an initial demyelinating event suggestive of MS or not treated until confirmation of MS.Data from study BENEFIT was used to model EDSS progression over time and transitions from CIS to MS. Exacerbations (relapses) were estimated from BENEFIT and published natural history data. Following transition to MS, all patients were assumed to be treated with IFNB-1b until EDSS 6.5. Direct and indirect medical costs of MS treatment and IFNB-1b were estimated from published literature and pricing schedules. Patient utilities were derived from EQ-5D data , supplemented by published data defined by EDSS score and relapse occurrence. Mortality was estimated using life tables and EDSS data. Costs (€2008) and outcomes were discounted at 3% per annum. Sensitivity analyses were performed on all key model parameters. RESULTS: Use of IFNB-1b was associated with slower EDSS progression (longer time to MS diagnosis), and reduced relapse burden. In the base case (50-year time horizon), ICUR of IFNB-1b versus no treatment was €20,500 / QALY gained using Poser diagnostic criteria and dominates using McDonald diagnostic criteria . Findings were sensitive to simulation time horizon, IFNB-1b cost and treatment effect (underlying rate of disease progression). CONCLUSIONS: IFNB-1b - treatment of patients with a CIS appears to offer reasonable value for money relative to many well accepted health care interventions.