We evaluated oligoclonal IgG band (OCB) patterns obtained by analyzing paired cerebrospinal fluid (CSF) and serum samples of 77 patients with acute demyelinating encephalomyelitis (ADEM) and 411 patients with multiple sclerosis (MS). OCBs were searched with isoelectric focusing and capillary immunoblotting. CSF-restricted OCBs were found in 89% of MS patients and 10% of ADEM patients (p<0.0001). Identical serum and CSF OCBs ('mirror pattern'), or no OCBs, were detected in 10% of MS patients and 84% of ADEM patients (p<0.0001). OCBs were also analyzed in 27 mice with proteolipid protein-induced experimental autoimmune encephalomyelitis (EAE). In this animal model, the 'mirror pattern' was the most frequently detected pattern (74%), with the immunizing antigen being the main OCB target. These results indicate that CSF analysis can help differentiate between MS and ADEM and that, similarly to EAE, the 'mirror pattern' observed in ADEM accounts for a predominantly systemic immune activation.