Entrapment of the posterior interosseous nerve is an uncommon syndrome characterized by an inability to extend the fingers and thumb. This entity bas become recognized increasingly since reported in 1966 by Capener and Muholland et al. The pathogenesis could be explained as an anatomical basis. And various cases were reported that trauma, local swelling, inflammation, postural and iatrogenic factors were concerned with the causes. In recent 4 cases of posterior interosseous nerve syndrome were experienced at Busan National University Hospital and following result were obtained.1l. Arcade of Frohse, fibrous band, recurrent radial artery are important compressive factors. 2. Surgical decompression led to an effective cure. 3. Tendon transfer is indicated after severe degenerative change.