The purpose of this study is to define the prevalence of peripheral nerve injury associated with full-thickness tears of the rotator cuff presenting with shoulder muscle atrophy. Twenty-five patients with the diagnosis of full-thickness rotator cuff tear were included. Electrodiagnostic testing, including nerve conduction studies and needle examination, was performed on all patients. There were 7 abnormal electromyographic examinations. The most common diagnosis was upper trunk brachial plexopathy severely affecting the axillary nerve (4/7), followed by suprascapular neuropathy (2/7) and cervical radiculopathy (1/7). The prevalence of associated peripheral neuropathy was found to be 28%. Greater degrees of atrophy were significantly associated with the presence of neuropathy in these patients. Careful neurologic screening in all patients and electromyographic examination in clinically suspicious groups are recommended in patients with full-thickness cuff tears before surgical repair.