As the effectiveness of continuous azathioprine therapy in multiple sclerosis (MS) is still controversial, we have conducted a prospective trial of the drug (2 mg/kg daily for at least 2 years) in patients with clinically definite MS, ie with remitting, remitting-progressive and progressive MS. At the end of an average 3-year follow-up 65% of the 40 patients with remitting-progressive had the same EDSS score as they had to start with. This was observed mainly in patients with longstanding MS and a high baseline EDSS. Of the 22 patients with a remitting course 77% showed no clinical deterioration and the annual relapse rate declined, although not significantly. In only one case did the remitting disease become remitting-progressive. Azathioprine would seem to have some efficacy in delaying the progression of remitting MS.