Thirty-nine patients with a clinical diagnosis of the pronator teres syndrome were seen during a seven-year period. They typically complained of aching discomfort in the forearm, weakness in the hand, and numbness in the thumb and index finger. Cyclic stress usually brought on the symptoms. The distinctive physical finding was tenderness over the proximal part of the pronator teres, which was aggravated by resisted pronation of the forearm, flexion of the elbow, and occasionally by resisted contraction of the flexor superficialis of the long finger. Electrophysiological testing of the median nerve showed abnormalities in a few patients, but localization of the abnormality was possible only rarely. Intraoperative recordings showed some improvement shortly after release of the median nerve in six of the ten forearms that were tested. Surgical exploration of thirty-six forearms in thirty-two patients showed intramuscular tendinous bands in the pronator, indentation of the muscle belly of the flexor superficialis in most forearms. Vascular and muscular abnormalities were seen occasionally. Of the thirty-six operations, twenty-eight gave good or excellent results; five, fair; and in three patients the symptoms were unchanged. The cause of failure was either inadequate decompression or misdiagnosis.