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Concomitant phrenic nerve block frequently occurs after brachial plexus block procedures in the neck and can result in substantial morbidity. In this study we sought to establish the anatomic basis using ultrasound imaging.We scanned the neck region of 23 volunteers with high resolution ultrasonography and identified the phrenic nerve in 93.5% of scans.The phrenic nerve was monofascicular with a mean diameter of 0.76 mm. The phrenic nerve position was nearly indistinguishable from the C5 ventral ramus at the level of the cricoid cartilage (mean distance 1.8 mm). Separation between the phrenic nerve and the brachial plexus increased substantially at more caudal levels in the neck. Phrenic nerve identification was confirmed by percutaneous injection of methylene blue followed by open dissection in a cadaver. Furthermore its identity was confirmed by ultrasound-guided transcutaneous nerve stimulation.This descriptive study found that the phrenic nerve and brachial plexus are within 2 mm of each other at the cricoid cartilage level, with additional 3 mm separation for every cm more caudal in the neck. Clinical trials with imaging guidance are needed to establish whether brachial plexus selective blocks can be consistently achieved above the clavicle.