This study was designed to identify clinical and electrodiagnostic indicators of prognosis in Guillain-Barré syndrome (GBS). Sixty severe GBS patients (all bedfast, 22 ventilator dependent) were analyzed clinically and with standard electromyography and nerve conduction studies. Both ventilator dependence and rapid evolution of weakness were more common in patients with a poor prognosis. The most powerful predictor of a poor outcome was reduced mean compound muscle action potential amplitude (less than 10% of the lower limit of normal).