Isoprinosine was used under double-blind, randomised, and placebo-controlled conditions in 52 patients with relapsing/remitting or progressive multiple sclerosis. All patients received pulsed treatment with methylprednisolone. There was no significant effect of treatment on clinical disability or the accumulation of MRI abnormalities, after correction of results for multiple comparisons. It is concluded that isoprinosine is not effective therapy for multiple sclerosis.