A multicentre, randomised, double-blind, placebo controlled study to evaluate the efficacy and safety of 4.5 and 9.0 MIU recombinant human interferon alfa-2a (Roferon-A) given thrice weekly in patients with relapsing-remittent multiple sclerosis is described. The patients are treated for 6 months followed by a 6 months drug-free period. The primary objective is to determine new disease activity analysed by monthly MRI with gadodiamide (GdDTPA-BMA, Omniscan). The study is conducted at eight centers in Norway and is completed in January 1996.