A cause for spontaneous paralysis of the posterior interosseous nerve has been sought for many years. In recent times an increasing number of reports on this subject has appeared. This paper describes a probable anatomical factor, the arcade of Frohse, in non-traumatic progressive paralysis of the posterior interosseous nerve. Though this arcade, described by Frohse (Frohse and Fraenkel 1908), the posterior interosseous nerve passes to enter the plane between the two heads of the supinator muscle. In this case, the arcade of Frohse may well play in causing progressive paralysis of the posterior interosseous nerve, with old anterior dislocation of the radial head. Paralysis of the muscles supplied by this nerve with no evidence of recovery should be treated by exploration and splitting of the arcade of Frohse.