This manuscript describes the outline of a double-blind, placebo-controlled, (European), multicentre phase III study to evaluate the safety and efficacy of 8 MIU of interferon beta-Ib given subcutaneously every other day for 3 years in patients with secondary progressive multiple sclerosis. The primary efficacy variable of this trial is the time to confirmed neurological deterioration as documented by the Expanded Disability Status Scale. The essentials of the study design are presented, including the rationale for the performance of the study and the selection of both clinical and magnetic resonance imaging outcome parameters.