We performed electrodiagnostic tests on 113 consecutive patients with acute Guillain-Barré syndrome (103 within 3 weeks of onset). The most common motor conduction abnormalities were proximal conduction block alone (27%), proximal block associated with a distal lesion (27%), and generalized slowing (22%). Other combinations of abnormalities each occurred in fewer than 10% of patients. Thirty-seven percent of patients initially had normal sensory nerve conduction study results, most often in association with proximal conduction block. The characteristic early electrodiagnostic changes in Guillain-Barré syndrome were often present when cerebrospinal fluid protein concentration was still normal. Extensive early fibrillations occurred in 10 patients, 6 of whom recovered well. Patients with early generalized slowing of motor nerve conduction velocity, combined abnormalities, or low muscle action potential amplitudes in ulnar, median, and peroneal nerves generally, but not always, had poorer outcomes than patients with conduction block in one nerve segment. There was no consistent relationship between results of electrophysiologic studies and overall clinical grade or limb power, except that none of the patients with an isolated proximal block had virtual or complete paralysis in the same limb.