We have analyzed immunoglobulin (Ig) isotypes and IgG subclasses in cerebrospinal fluid (CSF) and serum of patients with multiple sclerosis (MS) and other neurological diseases to determine whether different Ig isotype patterns correlate with clinical or paraclinical findings and CSF B cell populations. Intrathecal IgG1 synthesis was elevated in MS patients. An increased intrathecal IgM production was found in patients with a higher cerebral MRI lesion burden, whereas other clinical and paraclinical parameters were not associated with a specific Ig isotype or subclass profile. Finally, intrathecal IgG production (IgG1 and IgG3) correlated with the presence of mature B cells and plasma blasts.