In multiple sclerosis (MS) myelin damage is the result of a chronic inflammatory process mediated by CD4 positive T helper/effector cells. In experimental allergic encephalomyelitis (EAE), the animal model of MS, treatment with anti-CD4 antibodies can prevent the onset of disease. Natural history studies have demonstrated that gadolinium enhanced magnetic resonance imaging (MRI) of the brain is more sensitive and objective in assessing inflammatory disease activity in MS than clinical monitoring, so that less patients and shorter studies suffice to reach the same statistical power as compared to trials using clinical outcome parameters. In this paper we describe the design of an exploratory trial of chimeric monoclonal anti-CD4 antibodies in the treatment of MS. For this study we chose the number of active MS lesions on monthly gadolinium enhanced MRI scans as the primary outcome measure.