Posterior interosseous nerve palsies. One hundred seventy patients with radial nerve disorders were reviewed at the Louisiana State University Medical Center over a 15-year period. Of these, 32 had involvement of the posterior interosseous nerve exclusively. Findings included weak wrist extension with a radial drift, inability to extend the fingers, paralysis of thumb extension, and weak thumb abduction. Causes included entrapment at the arcade of Froshe (14 patients), laceration (6 patients), fracture (6 patients), compression or contusion (3 patients), and loss associated with tumor (3 patients). The ratio of men to women was 2:1, and the right arm was involved twice as often as the left. Preoperative evaluation included physical examination, electrophysiological testing (electromyogram/nerve conduction velocity), and roentgenograms of the elbow and forearm. Of the 30 patients (2 patients had bilateral lesions), 26 underwent operation. In the operative series, all 28 nerves had a function of Grade 3 or more of a possible 5 after 4 years of follow-up. Seventeen had achieved Grade 4/5, and 7 had obtained Grade 5/5. At operation, 23 nerves were found to be in continuity. Fourteen lesions of nerves in continuity were associated with entrapment and, not unexpectedly, transmitted a nerve action potential with slowed conduction and low amplitude across the lesion. Four nerves in continuity that had lesions caused by injury had nerve action potentials and were treated by neurolysis, and another 4 had no nerve action potentials and were treated by graft or suture repair. Five injured nerves were not in continuity. Two could be repaired by end-to-end suture, and 3 required graft repair. A large ganglion cyst involving the posterior interosseous nerve was also resected. 