The authors present 280 patients operated on for thoracic outlet syndrome (TOS). In a first group of patients anatomical variants were the striking findings. The underlying factor for TOS development is therefore a well defined structural condition and its pathogenetic mechanism is known to be a nerve fibre compression. In a second group there was no specific salient finding but a postural deviation. The unique pathological features were adhesions of the brachial plexus to the scalenus muscle. Consequently its pathogenetic mechanism is generally recognized as nerve fibre distraction. In all patients neurological, vascular and myofascial pain symptoms were observed before the operation. Neurological and vascular pain disappeared after surgery, while the myofascial pain remained. The authors believe that especially in the second, larger group of patients enhancement of the pain-immobility-fibrosis loop is the central pathogenetic factor on which surgical therapy is successful, and that myofascial hemisyndrome--probably arising from a long-standing postural deviation--is not a TOS dependent symptom. In TOS, therefore, there is a pain loop that cannot be resolved by surgical therapy alone. The connection between myofascial pain syndrome and TOS might explain the many controversial opinions regarding frequency, results and surgical possibilities of this lesion.