Background: The Controlled High Risk Subjects Avonex Multiple Sclerosis Prevention Study (CHAMPS) showed that IM interferon β - 1a (IFNβ - 1a) significantly slows the rate of development of clinically definite multiple sclerosis (CDMS) over 2 years in high- risk patients who experience a first clinical demyelinating event. This report highlights the primary results of a 5- year, open- label extension of CHAMPS (the Controlled High Risk Avonex Multiple Sclerosis Prevention Study in Ongoing Neurologic Surveillance CHAMPIONS Study). Objective: To determine if the benefits of IFNβ - 1a observed in CHAMPS are sustained for up to 5 years. Methods: CHAMPS patients at participating CHAMPIONS sites were enrolled in the study. All patients were offered, but not required to take, IFNβ - 1a 30 μ g IM once weekly for up to 5 years (from CHAMPS randomization). Patients who received placebo in CHAMPS were considered the delayed treatment (DT) group, and patients who received IFNβ - 1a in CHAMPS were considered the immediate treatment (IT) group. The primary outcome measure was the rate of development of CDMS. Additional outcomes included disease state classification at 5 years, annualized relapse rates, disability level at 5 years (Expanded Disability Status Scale), and MRI measures at 5 years. Results: Fifty- three percent (203/383) of patients enrolled in CHAMPIONS (n = 100, IT group; n = 103, DT group) and 64% (32/50) of CHAMPS study sites participated in CHAMPIONS. The median time to initiation of IFNβ - 1a therapy in the DT group was 29 months. The cumulative probability of development of CDMS was significantly lower in the IT group compared with the DT group (5- year incidence 36 ± 9 vs 49 ± 10% ; p = 0.03). Multivariate analysis suggested that the only factors independently associated with an increased rate of development of CDMS were randomization to the DT group and younger age at onset of neurologic symptoms. Few patients in either group developed major disability within 5 years. Conclusions: These results support the use of IM interferon β - 1a after a first clinical demyelinating event and indicate that there may be modest beneficial effects of immediate treatment compared with delayed initiation of treatment. Background: The Controlled High Risk Subjects Avonex Multiple Sclerosis Prevention Study (CHAMPS) showed that IM interferon β - 1a (IFNβ - 1a) significantly slows the rate of development of clinically definite multiple sclerosis (CDMS) over 2 years in high- risk patients who This report highlights the primary results of a 5-year, open-label extension of CHAMPS (the Controlled High Risk Avonex Multiple Sclerosis Prevention Study in Ongoing Neurologic Surveillance CHAMPIONS Study). Objective: To determine if the benefits All patients were offered, but not required to take, IFNβ - 1a 30 μg IM once weekly for up to 5 years (from CHAMPS randomization). Patients who received placebo in CHAMPS were considered the delayed treatment (DT) group, and patients who received IFNβ-1a in CHAMPS The primary outcome measure was the rate of development of CDMS. Additional outcomes included disease rate classification of 5 years, annualized relapse rates, disability level at 5 years (Expanded Disability Status Scale), and MRI measures at 5 years. Results: Fifty- three percent (203/383) of patients enrolled in CHAMPIONS (n = 100, IT group; n = 103, DT group) and 64% (32/50) of CHAMPS study sites participated in The median time to initiation of IFNβ-1a therapy in the DT group was 29 months. The cumulative probability of developing of CDMS was significantly lower in the IT group compared with the DT group (5-year incidence 36 ± 9 vs 49 ± 10%; p = 0.03). Multivariate analysis suggested that the only factors independently associated with an increased rate of development of CDMS were randomization to the DT group and younger age at onset of neurologic symptoms. Few patients in either group group major major disabili ty within5 years. Conclusions: These results support the use of IM interferon beta - 1a after a first clinical demyelinating event and that that there may be modest beneficial effects of immediate treatment compared with delayed initiation of treatment.