Background: Surgical stabilization of the sternoclavicular joint (SCJ) is infrequent, and cardiothoracic surgery assistance is often recommended. Patient safety and surgeon efficiency may be improved by greater understanding of the anatomic relationships near the SCJ. The purpose of this study is to determine the distances from the SCJ to critical structures in the superior mediastinum. Materials and methods: Distances from the posterior SCJ to adjacent mediastinal structures were recorded using contrast computed tomography scans of 49 consecutive patients. Patient sex, height, body mass index, side, age, and thickness of the sternum and medial clavicle were also recorded. Results: The mean distance to the nearest anatomic structure deep to the clavicular region of the SCJ was 6.6 mm and was 12.5 mm for the sternal region. The clavicle was an average thickness of 18 mm, and the sternum was an average thickness of 17 mm. The closest structure was the brachiocephalic vein. An artery was identified as the closest structure in 21.2% of patients. Distance differences between the right and left sides were noted, but sex had no bearing on distance to structures. Conclusion: Multiple mediastinal structures are close to the SCJ. The most frequent structure at risk of injury deep to the SCJ is the brachiocephalic vein. Such knowledge may improve patient safety. © 2013 Journal of Shoulder and Elbow Surgery Board of Trustees.