The onset of multiple sclerosis (MS) in 85% of young adults is with a subacute clinically isolated syndrome (CIS) of the optic nerves, brainstem, or spinal cord. Whereas multifocal brain lesions are present on MRI in many patients with a CIS, some patients have additional abnormalities on quantitative MRI in otherwise normal-appearing white and grey matter that suggest an extensive pathological process. Functional outcome for patients with symptomatic CIS lesions is determined by the interplay of inflammation, demyelination, axonal damage, remyelination, and cortical adaptation. Recovery of function may be accelerated by high dose corticosteroids, and although interferon beta delays the development of a second relapse, its long-term effect is unknown. A better understanding of pathological and pathogenetic processes in patients with a CIS will facilitate the development of disease-modifying treatments for patients with MS before they become disabled. Continued clinical and laboratory investigation of patients with a CIS should be encouraged.