A 23-year-old man was admitted to our hospital because of numbness in toes and finger tips. Within the next 72 hours general muscular weakness progressed so rapidly that he could not walk and necessitated a respirator. From the 18th hospital day plasmapheresis resumed 4 times every other day, which was followed by methylprednisolone pulse therapy (1,000 mg/day) for 3 successive days. The combination treatment described above brought about a dramatic recovery. Throughout the entire clinical course, enzyme immunoassay (serum) and enzyme-linked immunosorbent assay (CSF) to cytomegalovirus (CMV) were carried out serially. These results supported that he developed Guillain-Barré syndrome associated with CMV infection. Electrophysiological studies of the median and ulnar nerves were also performed serially up to 6 months after the onset. Based on the results the following comments were made; (1) In acute phase the amplitude markedly reduced with relative preservation of the distal latency, suggesting the presence of conduction block and mild demyelination. (2) Plasmapheresis, if done early enough, could prevent forthcoming secondary axonal degeneration probably by removing unknown nerve conduction blocking agents. (3) The degree of the distal latency prolongation at early phase was not a useful indicator for predicting the prognosis.