We studied natural killer (NK) cell activity and NK cell number in peripheral blood of 20 patients with Guillain-Barré syndrome (GBS). NK cell activity obtained within 7 days after onset of neuropathic symptoms in patients with GBS was significantly decreased compared with that in 20 control subjects. NK cell activity was not well correlated with disease severity. After plasmapheresis, NK cell activity recovered to the normal range. Two-color flow cytometry showed that the NK cell subsets studied were quantitatively similar in GBS patients and controls.