We report 6 patients with true neurogenic thoracic outlet syndrome. All were female and presented unilateral severe atrophy of the thenar muscles. Pain in the affected upper limb was frequently reported, but sensory deficit was slight or absent. Reduced amplitude of ulnar and median compound muscle action potential associated with a normal amplitude of median sensory nerve action potential (SNAP) and a reduced amplitude of ulnar SNAP was indicative of a chronic axon loss in the lower trunk of the brachial plexus. The absence of the medial antebrachial cutaneous SNAP in 5 patients and a reduction in amplitude compared to the unaffected side in the other patient indicated a C8-T1 postganglionic radicular lesion or a lower brachial plexus neuropathy. Radiography showed a rudimentary bilateral cervical rib or an elongated C7 transverse process in all cases. Surgery was performed, and in each case the lower part of the brachial plexus was found to be stretched and angulated over a fibrous band, which was removed. Pain was relieved after 1-4 weeks, but at 1 year, there was only minimal motor improvement and the electrophysiological results were unchanged.