We present an unusual constellation of the CSF/serum biomarkers of Guillain-Barré syndrome caused by unsteady-state condition of protein diffusion induced by plasmapheresis. Low blood concentrations of immunoglobulins were accompanied by their normal concentrations in the CSF, and hence the corresponding CSF/serum quotients seemed 'increased' suggesting intrathecal humoral response. This was in contrast to the results of isoelectrofocusing showing identical IgG bands pattern in the CSF and serum. Control lumbar puncture performed 6 weeks after the cessation of plasmapheresis, revealed normalization of the immunoglobulins' quotients. It must be stressed that the results of the CSF/serum analysis performed under non-steady state condition may be easily misinterpreted, and only considering the whole pattern of the CSF/serum biomarkers can assure correct interpretation of the results.