We describe a 36 year old woman with progressive bila teral ulnar neuropathy. Sonographic and magnetic resonance imaging studies revealed extensive focal ulnar nerve enlargement at the elbow. Histological studies gave evidence of an intraneural perineurioma. Because intraneural perineurioma usually appears as a single mass lesion at sites other than typical entrapment sites, this mode of presentation is unusual. We discuss the nature of this benign tumor and the differential diagnosis of nerve enlargement. Knowledge of possible causes of nerve thickening is crucial when performing imaging in patients with neuropathies. We describe a 36 year old woman with progressive bila teral ulnar neuropathy. Histological studies gave evidence of an intraneural perineurioma. Because intraural perineurioma usually appears as a single mass lesion at sites other than typical entrapment sites, this mode of presentation is unusual, We discuss the nature of this benign tumor and the differential diagnosis of nerve enlargement. Knowledge of possible causes of nerve thickening is crucial when performing imaging in patients with neuropathies.