Background: The EVIDENCE (Evidence of Interferon Dose-Response: European Nort h American Comparative Efficacy) Study demonstrated that patients with multiple sclerosis (MS) who initiate interferon beta-1a therapy with 44 μg 3 times week ly (TIW) were less likely to have a relapse or activity on magnetic resonance im aging (MRI) compared with those who initiate therapy at a dosage of 30 μg 1 tim e weekly (QW). Objective: To determine the effect of changing the dosage from 30 μg QW to 44 μg TIW in this extension of the EVIDENCE Study. Design/Patients: Patients with relapsing MS originally randomized to interferon beta-1a, 30 μg QW, during the comparative phase of the study changed to 44 μg TIW, whereas pat ients originally randomized to 44 μg TIW continued that regimen. Patients were followed up, on average, for an additional 32 weeks. Main Outcome Measure: The w ithin-patient pretransition to posttransition change in relapse rate. Results: At the transition visit, 223 (73%) of 306 patients receiving 30 μg QW converte d to 44 μg TIW, and 272 (91%) of 299 receiving 44-μg TIW continued the same therapy. The posttransition annualized relapse rate decreased from 0.64 to 0.32 for patients increasing the dose (P < .001) and from 0.46 to 0.34 for patients c ontinuing 44-μg TIW (P=.03). The change was greater in those increasing dose a nd frequency (P=.047). Patients converting to the 44-μg TIW regimen had fewer active lesions on T2-weighted MRI compared with before the transition (P=.02), whereas those continuing the 44-μg TIW regimen had no significant change in T2 active lesions. Patients who converted to high-dose/high-frequency interferon beta-1a therapy had increased rates of adverse events and treatment terminatio ns consistent with the initiation of high-dose subcutaneous interferon therapy. Conclusions: Patients receiving interferon beta-1a improved on clinical and MR I disease measures when they changed from 30 μg QW to 44 μg TIW. Background: The EVIDENCE (Evidence of Interferon Dose-Response: European North American Comparative Efficacy) Study demonstrated that patients with multiple sclerosis (MS) who were interferon beta-1a therapy with 44 μg 3 times week ly (TIW) were less likely to have a relapse or activity on magnetic resonance im aging (MRI) compared with those who orients on a dosage of 30 μg 1 tim e weekly (QW). Objective: To determine the effect of changing the dosage from 30 μg QW to 44 μg TIW in this extension of the EVIDENCE Study. Design / Patients: Patients with relapsing MS originally randomized to interferon beta-1a, 30 μg QW, during the comparative phase of the study changed to 44 μg TIW, pat pat originally originally randomized to 44 μg TIW continued that regimen. Patients were followed up, on average, for an additional 32 weeks. Main Outcome Measure: The w ithin-patient pretransition to posttransition change in relapse rate. Results: At the transition visit, 223 (73%) of 30 6 patients receiving 30 μg QW converte to 44 μg TIW, and 272 (91%) of 299 receiving 44-μg TIW continued the same therapy. The posttransition annualized relapse rate decreased from 0.64 to 0.32 for patients increasing the dose (P <. 001) and from 0.46 to 0.34 for patients c ontinuing 44-μg TIW (P = .03). The change was greater in those increasing dose a nd frequency (P = .047). Patients converting to the 44-μg TIW regimen had fewer active lesions on T2-weighted MRI compared with before the transition (P = .02), but those continuing the 44-μg TIW regimen had no significant change in T2 active lesions. Patients who converted to high-dose / high-frequency interferon beta-1a therapy had increased rates of adverse events and treatment terminatio ns consistent with the initiation of high-dose subcutaneous interferon therapy. Conclusions: Patients receiving interferon beta-1a improved on clinical and MR I disease measures when they changed from 30 μg QW to 44 μg TIW.