• We attempted to identify predictive factors of early beneficial response to plasmapheresis in Guillain-Barré syndrome (GBS). We reviewed 24 patients with typical severe GBS who underwent plasmapheresis and analyzed their outcome at one month. One group of 14 patients, designated as responders, improved dramatically, while ten patients showed little response. Age was the only important clinical predictor, with responders being younger. No other clinical variable (sex, preceding illness, severity, timing of plasmapheresis, cranial nerve involvement, or cerebrospinal fluid findings) reached significance. Among electrophysiologic parameters obtained before plasmapheresis, the amplitudes of compound muscle action potentials with distal stimulation of median and peroneal nerves were significantly reduced in nonresponders. Plasmapheresis may improve only a subgroup of patients with GBS. Among patient characteristics, age and amplitudes of compound muscle action potentials are important predictors of early responsiveness.