Guillain-Barré syndrome (GBS) is an acute, post-infectious, inflammatory, autoimmune peripheral neuropathy with a highly diverse clinical course and outcome. We classified GBS on the basis of patients' first nerve conduction and validated this system to be associated with outcome on the basis of electrophysiological characteristics during the acute phase of GBS. We retrospectively evaluated 40 GBS patients who underwent their first electrophysiological study within 14 days of onset and classified GBS into four patterns: (1) acute inflammatory demyelinating polyneuropathy (AIDP) pattern with sensory nerve conduction abnormalities (motor-sensory AIDP: MS-AIDP), (2) AIDP pattern without sensory nerve conduction abnormalities (motor AIDP: M-AIDP), (3) acute motor axonal neuropathy (AMAN) pattern, and (4) minor abnormalities pattern. We compared the clinical, electrophysiological, and laboratory findings between groups and determined subgroups associated with poor outcome. The MS-AIDP and AMAN patterns more frequently exhibited prolonged recovery compared with the M-AIDP and minor abnormalities patterns and were associated with prolonged recovery (specificity, 100%; sensitivity, 73%; P < 0.001). The period of inability to walk independently was significantly longer in the MS-AIDP and AMAN patterns than in the M-AIDP and minor abnormalities patterns (median 85 vs. 10 days; P < 0.001). In conclusion, our classification of GBS using a single nerve conduction study in the early phase of disease is associated with outcomes. This classification can be used to counsel individual patients and guide decision-making with respect to treatment.