Whether the conduction abnormalities and lesions underlying them are randomly distributed throughout the peripheral nervous system (PNS) or certain regions are selectively more vulnerable to attack is an unresolved question in Guillain-Barré syndrome (GBS). To examine this question, 15 cases of GBS were comprehensively examined using electrophysiological techniques which allowed close examination, quantitation and comparison of conduction abnormalities in motor and sensory fibres of the upper limb between the spinal cord and the distal extremities of the nerve fibres. Comparison of these studies with results expected in a model where the chances of conduction failure were uniformly distributed led to the conclusion that conduction slowing and block were not uniformly distributed in most cases. Conduction block was maximal in the terminal segment distal to the wrist and to a lesser extent both conduction block and conduction slowing were disproportionately greater across the elbow and in the axillary to spinal root segments in over one half of the cases. These findings support the hypothesis that certain regions, perhaps because of relative deficiencies of the blood-nerve barrier, may be more vulnerable in GBS than other regions.