THE THORACIC outlet syndrome is a well-known clinical entity. Reports of neurovascular compression by a cervical rib, the scalenus anticus muscle, deformity of the clavicle, and other anatomical variations have appeared frequently. Although anomalous first ribs also distort the thoracic outlet, they have only rarely appeared in the literature as a cause of the syndrome. Two cases of thoracic outlet syndrome associated with anomalous first ribs are presented. Report of Cases Case 1.— A 32-year-old white man was admitted to the hospital because of pain in the left shoulder and hand. He stated that when he was 20 years old he became aware of a painful mass in the left supraclavicular space. This mass was hard and tender, and pain in the region of the mass was associated with pain in the left axilla and along the medial aspect of his left arm. The pain descended down his left forearm