Personal evaluation of more than 2,300 patients for possible thoracic outlet syndrome (TOS) and knowledge gained from 980 TOS operations in 766 patients (operative incidence of 33.7 per cent of the patients examined) have shown that most patients with TOS have anomalous fibrous muscular bands near the brachial plexus that predispose them to neurologic irritation or compression involving the plexus. Anatomic analysis during operations for TOS, plus cadaver dissections, have disclosed seven distinct types of fibromuscular bands in addition to the less frequent bony anomalies long associated with neurovascular compression. One third of fifty-eight cadaver thoracic outlets dissected showed at least one of the seven muscular anomalies recognized at operations. These anomalies can be accurately related to the patients' symptoms, which are neurologic complaints in 99 per cent of the patients examined who ultimately have the diagnosis of TOS established. Neurologic symptoms are clearly explained by the anomalous bands irritating or compressing the brachial plexus and rarely have any effect on the subclavian vessels. These studies, and others before, have shown no correlation with impairment of circulation or positional radial pulse changes in almost all patients with true TOS. Also, arteriograms and nerve conduction studies generally have failed to be of value in establishing the accurate diagnosis. Reasons for these conclusions are explained, and the most reliable tests are described. The most effective means of relief of severe symptoms of TOS is to alter the mechanical irritation or compression of the brachial plexus by completely resecting the first throacic rib and all anomalous fibromuscular tissue around the plexus and subclavian vessels. If patients are are throughly evaluated with appropriate tests and highly selected for surgical treatment, gratifying relief will result in more than 90 per cent of patients, if the correct operation is performed with meticulous technic.