Ten patients with wasting of hand muscles and sensory symptoms mainly in the ulnar nerve area were diagnosed as having a thoracic outlet syndrome and were operated on for it. Sensory action potentials (SAPs) from digit 5 were reduced in amplitude in eight patients and those from digit 3 in three patients, and the SAP amplitude ratio between digits 3 and 1 was decreased in five patients. Sensory and motor conduction velocity was normal in all. Quantitative electromyographic studies of ulnar and median innervated small hand muscles revealed changes compatible with chronic partial denervation in most patients. The combined findings of chronic partial denervation of the small hand muscles, decreased SAP amplitude from digit 5 and sometimes also from digit 3, and normal motor and sensory conduction velocity are compatible with a compression of the C-8 and T-1 roots or the lower trunk of the brachial plexus.