The efficacy of azathioprine in relapsing-remitting multiple sclerosis. We randomized 59 patients with relapsing-remitting multiple sclerosis to receive azathioprine (AZA) 3.0 mg/kg daily or placebo in a double-masked therapeutic trial. Analysis of data for predetermined primary outcome measures demonstrated a significant difference favoring AZA for observed mean exacerbation rate after 2 years of therapy and time to deterioration in both Ambulation Index and Kurtzke Expanded Disability Status Scale score. This study confirms a modest therapeutic benefit for azathioprine previously reported by other investigators. 