A rare case of acute posterior interosseous nerve palsy caused by septic elbow arthritis is reported. The nerve was compressed beneath the arcade of Frohse by hypertrophied synovium and joint fluid at the anterior aspect of the radial neck. Decompression of the nerve, synovectomy, and irrigation of the elbow joint were done. effective. Six months after the surgery the nerve palsy had recovered completely. Two years after surgery there was no recurrence of the infection or nerve palsy. (J Hand Surg 2003;28A:44-47.