Thoracic outlet syndrome (TOS) is one of the most poorly understood syndromes. Neurogenic TOS is found in 95% of cases. The described anatomical spaces transform and evolve into 'entrapment spaces'. The aetiology is unclear. This study was based on the observation by a single surgeon that there appeared to be a high incidence of anatomical abnormalities in patients with neurogenic TOS.To attempt to clearly define anatomical anomalies causing TOS.The records from a prospectively maintained computer database of 219 patients submitted for surgery over a 10-year period (1999-2009) were reviewed. A substudy was done on the patients operated on over the last 4 years (n=63) in whom details of the intraoperative anatomical findings were meticulously recorded.Over the last 4 years, the surgical findings in the last 63 patients (67 operations) revealed a significant number of anatomical abnormalities believed to be responsible for the nerve compression. Brachial plexus anomalies were found in 99% of the patients--the majority comprised the postfixed configuration. In addition, 58% had a soft-tissue anomaly, 27% had a bony anomaly and 3% had other abnormalities. The majority had combinations of these abnormal findings.These findings strongly suggest that there is usually an identifiable anatomical cause, typically the brachial plexus, for the symptoms of TOS. We strongly recommend that the supraclavicular approach be used in order to define anatomical aberrations. Brachial plexus configuration anomalies causing TOS have not been emphasised previously. Further detailed recordings of these findings may help us better understand the aetiology of this poorly defined syndrome.