There is an evident need for a quantitative laboratory marker for ascertaining disease activity and treatment effects in multiple sclerosis (MS) patients. Activity of the disease process in MS is accompanied by myelin breakdown and appearance of myelin basic protein (MBP) in cerebrospinal fluid (CSF). In this paper MBP in CSF of relapsing-remitting (RR) MS patients is reviewed. MBP in CSF is a fragment containing an epitope corresponding to amino acid residues 45-89 of the native molecule. From several relevant studies about CSF MBP in RR MS the following relations can be concluded: CSF MBP levels in active MS patients are frequently increased (45-100%), remain increased until 5 to 6 weeks after onset symptoms and are higher in polysymptomatic exacerbations and correlate with number of gadolinium-enhanced (Gd) lesions on MRI, severity of relapses, EDSS score and CSF intrathecal IgM synthesis. After an intravenous methylprednisolone treatment the increased CSF MBP levels return to normal values and reduction in CSF MBP is related to reduction in EDSS score, number of Gd lesions and CSF intrathecal IgM synthesis.