A case of ulnar nerve compression at the wrist within Guyon's canal is reported. The clinical presentation initially appeared consistent with an ulnar nerve entrapment at the elbow. The true diagnosis of an ulnar sensorimotor nerve lesion occurring within the canal of Guyon was made electrophysiologically. Magnetic resonance imaging demonstrated compression of the nerve within the canal by a ganglionic cyst, which was confirmed by surgical intervention. Ulnar nerve entrapment at the wrist is uncommon and difficult to diagnose; therefore, it is important to understand the nerve's anatomical course and distribution to allow for accurate diagnosis by clinical and electrodiagnostic evaluations. Electrodiagnosis is an important tool in identifying ulnar nerve lesions at the wrist while excluding other disorders in the differential and recognizing coexisting pathology.