Intrathecal IgM synthesis was assessed through the detection of oligoclonal bands in cerebrospinal fluid (CSF) and through calculation of IgM index values and total CSF IgM amounts. Intrathecal production of IgM was then evaluated along with other CSF parameters and clinical variables in 150 patients with multiple sclerosis (MS). Intrathecal production of IgM was detected in 55% of patients and was found to correlate with disease activity, manifesting as a recent relapse, as well as the total number of relapses. An inverse correlation was found between CSF oligoclonal IgM and both the time interval between the last relapse and lumbar puncture, and duration of the disease process. It also correlated with the number of leucocytes, free kappa and lambda light chain bands in CSF. No correlation was found between intrathecal IgM synthesis and degree of disability (Kurtzke scale), disease progression index (Poser scale), or total cerebral plaque burden (detected by enhanced CT and MRI scans). Oligoclonal IgM bands were found to be more specific for active disease process than either IgM index or total CSF IgM amount. It is concluded that intrathecal synthesis of IgM, as detected by CSF oligoclonal bands, is a useful parameter in monitoring disease relapse activity in MS.