Two recent properly controlled trials performed in patients with relapsing-remitting MS have demonstrated that interferon beta substantially improves the evolution of the disease. The results of the Optic Neuritis Treatment Trials suggest that early treatment of patients with isolated optic neuritis may delay the conversion to clinically definite MS (CDMS). Moreover, clinical trials in MS and in immune-mediated diseases indicate that better results are obtained in patients in the early phases of the disease. We present the design of a multicentre, randomized, double-blind, placebo-controlled study of recombinant interferon beta (Rebif-Serono) in patients with a first attack suggestive of MS. The primary objective of the study is to investigate the efficacy of Rebif, administered subcutaneously at the dose of 8 million international units (MIU) once a week for 2 years, on the risk of developing CDMS.