The aim of the study was to identify the most common electrophysiological abnormalities in early Guillain-Barré syndrome (GBS). Neurophysiological data on 51 GBS patients assessed within 12 days of symptom onset were reviewed. Acute inflammatory demyelinating polyradiculoneuropathy (AIDP) was present in 46 of 51 GBS patients. The following abnormalities were observed in our AIDP patients: absent H reflex in 90.7%, conduction block in the Erb-to-axilla segment in 78.6%, motor conduction velocity suggestive of demyelination in the Erb-to-axilla segment in 45.2%, prolonged F wave latency in 65.2%-73.8% of patients but only 20.0%-37.0% with prolonged F wave latency suggestive of demyelination, and reduced or absent sensory nerve action potential in 62% of patients. Abnormal values of terminal latencies, and motor and sensory conduction velocities in distal nerve segments suggestive of demyelination were recorded in less than 30% of patients. In conclusion, the most sensitive parameter in early GBS patients is conduction block in the most proximal segments of the peripheral nervous system, directly determined in the Erb-to-axilla segment or indirectly as absent H reflex. Motor conduction studies in the Erb-to-axilla segment are very informative in early GBS patients.