A 60-yr-old woman reported severe pain in the proximal part of her left forearm. The pain was also radiating toward the median nerve-innervated areas of the arm, especially during elbow flexion. Ultrasonographic imaging showed a well defined ganglion cyst around the distal biceps tendon close to the median nerve. Under ultrasound guidance, the cyst was aspirated and corticosteroid injection was performed. The patient's complaint improved 1 wk after the injection, and she started to flex her elbow without any pain. This case highlights the role of ultrasonography as a useful adjunctive tool not only to morphologically confirm a peripheral nerve entrapment but also to uncover the possible underlying etiology and to guide precisely during an intervention.