Ganglion cyst (GC) is a mucin-filled benign mass which is located adjacent to joints, ligaments, nerves and tendon sheaths.GC is one of the most common benign hand and wrist tumor.It can be seen at any age but, usually occurs in 20-40 years-old women.Although most of the GC in the wrist is on the dorsal side, it may also appear on the volar side up to 20%. 1 GC is usually asymptomatic, but it can cause swelling, pain, limitation of movement and difficulties in daily activities.If the GC compresses the surrounding structures such as arteries, veins, nerves or tendon sheaths, it may cause pain and interfere with certain activities. 2 The diagnosis of GC is based on patient's history and physical examination.When there is no obvious swelling, the ultrasonography (US) and magnetic resonance imaging (MRI) become important for detecting the location of the cyst.Ulnar nerve compression is the second frequent nerve entrapment seen in the upper extremity.Ulnar nerve may be compressed by a ganglion cyst with mechanical pressure.US can reveal deformities causing extraneural entrapment of nerves such as GCs, soft tissue tumors, fibrous bands, osseous pathologies. 3In this report, we presented a 40 yearold musician with Guyon neuropathy due to GC.