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Neuromuscular choristoma (NMC) is a rare benign peripheral nerve lesion characterized by mature skeletal or smooth muscle admixed with nerve fibers whereas lipomatosis of nerve (LN) is characterized by adipose tissue. We describe a case of a 49-year-old woman with a fat-containing mass associated with the brachial plexus appearing as a hibernoma, atypical LN, or atypical lipomatous tumor. Biopsy revealed pathognomonic histologic features of NMC. We suggest the 50% rule previously proposed to distinguish NMC from other fatty lesions be applied to the portion of the lesion with soft tissue elements, and not the lesion as a whole.