Forty patients, affected by multiple sclerosis with remitting-relapsing or progressive course, were included in a double blind study of treatment with azathioprine (2 mg/kg/day) lasting 3 years. The mean changes on the Expanded Disability Status Scale and in the survival analysis show a trend in favour of azathioprine both in slowing disease progression and reducing relapse frequency. These findings, repeatedly observed in similar trials, indicate that azathioprine should be used in the treatment of multiple sclerosis.