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Purpose: To study the ulnar nerve in fresh-frozen cadavers as related to endoscopic release of the cubital tunnel and to present a retrospective review of patients treated with decompression via endoscopic visualization., Methods: To further our understanding of relevant anatomy, we dissected 26 cadaver limbs. We paid special attention to fascial membranes as potential sites of constriction as well as the position of nerves, vessels, and aberrant anatomy. These findings facilitated our understanding of the extent of release in 80 patients (92 cases) with endoscopic cubital tunnel simple decompression. Outcome measures included Disabilities of the Arm, Shoulder, and Hand score, Gabel and Amadio score, and grip and pinch strengths., Results: We noted fascial bands proximal to the medial epicondyle in 12 of 26 cadaver specimens, 2 of which could be the so-called arcade of Struthers. We observed a high degree of variability in the anatomy of the flexor pronator aponeurosis distal to the medial epicondyle. Where present (n = 10), medial antebrachial cutaneous nerve branches crossed the ulnar nerve at an average distance of 2.9 cm from the medial epicondyle (range, 1.0-4.5 cm). Aberrant structures were noted in 8 of the 26 specimens, including an anconeus epitrochlearis muscle in 2 specimens, a basilic vein crossing the ulnar nerve in 4 specimens, and an accessory origin of the medial head of the triceps from the medial intermuscular septum in 2 specimens. In the clinical portion of this study, the average Disabilities of the Arm, Shoulder, and Hand score before surgery was 49 (n = 34) and after surgery was 25 (n = 56). The Gabel and Amadio outcome scores were 24 excellent, 40 good, 25 fair, and 3 poor (n = 92). Average follow-up was 8.2 months (range, 0.1-35 mo)., Conclusions: Cadaveric dissections shed light on vulnerable anatomical structures during release, including branches of the medial antebrachial cutaneous nerve, ulnar nerve, brachial artery, fascial bands, and basilic vein. The high degree of anatomical variability in this study highlights the advantage of endoscopic visualization in allowing surgeons to minimize surgical trauma., Type of Study/level of Evidence: Therapeutic IV., (Copyright © 2014 American Society for Surgery of the Hand. Published by Elsevier Inc. All rights reserved.)