The F-wave conduction velocity (FWCV) in the central segment (axilla to spinal cord) of the median and ulnar nerves was compared to the motor nerve conduction velocity (MNCV) in the more distal segments in nine patients with mild cases of the Guillain-Barré syndrome. In four patients, FWCV was slow despite normal or borderline MNCV. In four others, both FWCV and MNCV were abnormal. One patient showed slow MNCV with normal FWCV. The average FWCV was significantly decreased (40.9 +/- 12.0 meters/sec for median and 42.9 +/- 8.3 meters/sec for ulnar nerves) when compared to the corresponding normal values (64.3 +/- 6.4 meters/sec and 63.1 +/- 5.9 meters/sec). These data support the hypothesis that the central segment of a nerve is predominantly involved in some patients with Guillain-Barré syndrome.