Nerve sheath ganglion is a reactive lesion seen in response to irritation or trauma to a nerve. The most common location is the external popliteal nerve (common peroneal nerve) at the head of the fibula. It presents as a painful mass with numbness in the distribution of the affected nerve and, in extreme cases, with paralysis. The nerve shows myxoid change with cyst formation leading to a localized mass-like effect. The cyst may completely replace the nerve segment and push the nerve fascicles to the periphery (as seen in this case). The differential diagnosis is with nerve sheath myxoma. Treatment can consist of either local excision or decompression of the cyst.