Posterior interosseous nerve is a deep muscular branch of radial nerve, and non-traumatic paralysis of this nerve is relatively rare cases. Posterior interosseous nerve may be compressed by fibrous bands in front of the radial head, radial recurrent fan of vessels, arcade of Frohse, and sharp tendinous margin of the extensor carpi radialis brevis muscle.

We report two cases of non-traumatic paralysis of the posterior interosseous nerve which developed spontaneously.