Introduction: Approximately 85% of patients with multiple sclerosis have an initial demyelinating event. Treatment with interferon beta delays the progression of multiple sclerosis for nearly two years in patients with a clinically isolated syndrome. In Colombia, interferon is very expensive when compared to other countries.Objective: Purpose: We sought to determine the cost-effectiveness of a two-year interferon beta treatment within Colombia in patients with a clinically isolated syndrome.Materiales y methods: Based on patient and society perspectives, a cost-effectiveness analysis was conducted using a decision tree. A variety of probabilities were defined after a systematic review of the available literature. The disease costs were calculated by reviewing medical charts at the Hospital San Ignacio University and surveys completed by multiple sclerosis patients. To control for uncertainty in these data, analysis of approximately one-thousand patients was performed using Monte Carlo methods.Results: The two-year treatment cost per patient exceeds Col$ 95,000,000 (US$ 50,000). Approximately 80 % of this cost corresponds to medications (US$ 40,500). The price of relapse and indirect costs totals Col$ 41,632,149 (US$ 21,744) and Col$ 11,656,389 (US$ 6,088), respectively. Treatment represents an increase of 0.06 quality-adjusted life years (QALY). The incremental cost-effectiveness ratio exceeds the threshold, regardless of the use of Monte Carlo methods for analysis.Conclusion: Administering interferon beta over the course of two years to high-risk patients with a clinically isolated syndrome is not cost-effective within Colombia.