Posterior interosseous nerve palsy caused by a ganglion is not common and most previous patients were treated with excisional surgery. We treated a case conservatively with needle aspiration using ultrasonography, after a nerve conduction study. A 77-year-old man presented with impaired active finger extension of the left metacarpophalangeal joints. The nerve conduction study revealed conduction block of the left radial nerve near the elbow. Ultrasonography demonstrated a hypoechoic mass anterior to the radial neck compressing the posterior interosseous nerve. Then, needle aspiration of the mass was conducted under ultrasonography. Two months later, active finger extension recovered to normal. A ganglion can be diagnosed with ultrasonography and needle aspiration can be carried out safely under ultrasonography. A nerve conduction study can assess the degree of nerve damage. The combination of ultrasonography and a nerve conduction study can facilitate conservative treatment of needle aspiration for posterior interosseous nerve palsy caused by a ganglion.