In 13 of 14 patients with Guillain-Barré syndrome (GBS), we observed multiple A waves in at least one limb nerve on routine electroneurographic studies within 7 days after onset of symptoms. The patient without A waves had a severe axonal type of GBS with tetraplegia and almost complete loss of M responses following electrical stimulation of limb nerves. In the remaining 13 patients, on average 8 +/- 2 (mean +/- SD) A waves were present in each tibial nerve (n = 24) and 4 +/- 1 A waves in each peroneal nerve (n = 26). About half of the A waves were below 50 microV in amplitude, whereas amplitudes were higher than 120 microV in only 22 of 299 A waves. Of these A waves, 68 were not constantly elicitable. There was a significant correlation between the number of A waves up to 50 ms poststimulus and the reduction in amplitude of the compound muscle action potential when elicited with proximal compared to distal stimulation in the peroneal (n = 26, P < 0.0005; Kendall's tau) and tibial nerves (n = 24, P < 0.002). Therefore, in GBS both conduction block and A waves are presumably signs of inflammatory nerve lesions. The existence of multiple A waves soon after onset of symptoms seems to be a sensitive sign of GBS.