There is no "gold standard" for diagnosing thoracic outlet compression syndrome (TOS), however, anesthetic blocks of the anterior scalene muscle (ASM) have been used as a means of predicting which patients may benefit from surgical decompression. The standard technique of using surface landmarks often results in inadvertent somatic block and sympathetic block because there is no reliable verification of needle tip localization. The present study was undertaken to determine if needle tip localization can be improved by using electrophysiological guidance. ASM blocks were performed for patients with a diagnosis of possible TOS. An insulated hypodermic needle was inserted into the ASM which was identified during electromyogram (EMG) activation maneuvers. Stimulation was performed to make sure that the needle tip was not in the brachial plexus. Local anesthetic was instilled and the intensity of pain induced by TOS stress maneuvers was compared to pain ratings obtained after control injections. The ASM could be identified electromyographically in all 122 cases. There were no instances of inadvertent somatic block nor sympathetic block. Of 38 patients who underwent surgical decompression of the thoracic outlet, 30 of 32 (94%) with a positive block had a good outcome compared with 3 of 6 (50%) who underwent surgery in spite of a negative block. Electrophysiological guidance facilitates accurate needle tip placement in the performance of ASM blocks; the results of these blocks appear to correlate with surgical outcomes.