Guillain-Barré syndrome is a well-defined clinical entity corresponding to primary inflammatory demyelinating lesions of peripheral nerves and spinal roots in the majority of cases seen in Western Europe and North America. Documentation of conduction abnormalities characteristic of demyelination in the context of rapidly developing weakness confirms the clinical diagnosis of acute inflammatory demyelinating polyradiculoneuropathy. These abnormalities include (1) conduction block, (2) markedly prolonged distal latencies, (3) marked slowing of motor conduction velocities, and (4) absent or impersistent F responses. Other abnormalities are of ambiguous significance: (1) low amplitude compound muscle responses, (2) absent sensory nerve potentials, (3) reduction in the maximum EMG recruitment pattern, which may all (1, 2, and 3) indicate conduction block or axonal degeneration, and (4) fibrillation potentials due to breakdown of motor axons, a frequent non-specific effect of primary inflammatory demyelination. A Guillain-Barré variant due to immune-mediated primary axonal degeneration has also been recently described.