The increasing activity in sport lead especially in untrained persons to overstress of the elbow joint. Often one tends to diagnose a epicondlylits humeri radialis i.E. tennis elbow. In 1873, Runge (16) first described what we now call epicondylitis humeri radialis, i.e. what Morris (10) 1882 called "Tennis elbow." Its etiology is still under discussion, and the diagnosis of tennis elbow necessitates exact and detailed differential diagnostic efforts. In the work presented, we describe one major differential diagnosis, the posterior interosseous nerve syndrome or supinator entrapment syndrome. With sketches and anatomical preparations we describe the course of the radial nerve around the elbow joint and of its ramus profundus within the supinator muscle pouch. Listing representative causes of this syndrome, we also comment on clinical aspects and our surgical method of choice. We have interviewed and reexamined 6 of our patients suffering from a so called therapy-resistant epicondylitis humeri radialis. Due to their clinical picture all six had been operated for posterior interosseous nerve syndrome/supinator muscle entrapment syndrome. We present our results including neurophysiological evaluations. In relapsing, therapy resistant and diagnostically untypical humeral radial epicondylitis, a posterior interosseous nerve syndrome should be considered. After clinical and electromyographic examination a revision of the supinator channel is justified in selected cases.