A Fisher syndrome (FS) patient with antibody to tetrasyaloganglioside GQ1b (GQ1b) developed late limb weakness. Serial motor conduction velocities (MCVs) showed a marked reduction of distal compound muscle action potential (CMAP) amplitudes, worse at 2–3 weeks, followed by a dramatic increase in week 5. Motor conduction velocities were always in the normal range, distal motor latencies changed only slightly, and conduction block in intermediate nerve segments was absent. These electrophysiological data might suggest an axonal neuropathy or a distal demyelinating conduction block. However, the dramatic increase of distal CMAP amplitudes over a short time without significant changes of distal motor latencies, CMAP duration, and morphology indicate that weakness in this FS patient might be due to a block of acetylcholine release from motor terminals, possibly mediated by anti‐GQ1b antibodies. © 1999 John Wiley & Sons, Inc. Muscle Nerve 22: 640–644, 1999