Anatomic observations were made during 200 consecutive transaxillary surgical procedures performed in 175 patients because of unremitting signs and symptoms of nerve or vascular compression at the thoracic outlet. There were 160 cases of brachial plexus compression and 40 cases of arterial or venous occlusion. In 132 (66%) of these cases, single or multiple abnormalities were recognized that represented developmental variations previously described in anatomic dissections or in embryologic studies. There were 17 cases of cervical rib or first thoracic rib abnormalities and 20 supernumerary scalene muscles. Developmental variations were identified in 86 scalene and 39 subclavius muscles or their insertions. Comparisons revealed a higher percentage of developmental anomalies in this group of patients than in consecutive anatomic investigations reported in unselected populations. This information was interpreted in light of recent embryologic studies and histochemical and morphometric ultrastructural studies of scalene muscle. The preponderance of evidence suggests that neurovascular compression in the region of the thoracic outlet derives from a combination of these factors: predisposing morphologic variations, structural modifications conditioned by functional requirements, and changes in fiber type or myosin isoform consequent to trauma. The correlation of clinical syndrome with morphologic characteristics alone was significant only for the Paget-Schroetter syndrome.