The case described is thought to be the first report of ulnar neuropathy caused by a cystic myxomatous degeneration of fat necrosis. The 12-day preoperative duration of symptoms is also the shortest reported for nontraumatic ulnar neuropathy; the average period between onset of symptoms and operation is 10 months. At operation, the patient's nerve was found to be stretched severely over the tumor, which was excised, thus decompressing the ulnar nerve. The procedure relieved the patient's symptoms of tingling and numbness, and he remained asymptomatic at the 12-month follow-up examination.