Objective. To evaluate the diagnostic usefulness of provocative tests, Doppler ultrasonography, electrophysiological investigations, and helical computed tomography (CT) angiography in thoracic outlet syndrome (TOS). Patients and methods. We prospectively evaluated 48 patients with a clinical suspicion of thoracic outlet syndrome. Standardized provocative tests, an electromyogram and somatosensory evoked responses, a Doppler ultrasonogram, and a helical CT arterial and/or venous angiogram with dynamic maneuvers were done on each patient. The final diagnosis was established by excluding all other causes based on all available data. The agreement between the results of each investigation and the final diagnosis was evaluated. Results. Provocative tests had mean sensitivity and specificity values of 72% and 53%, respectively, with better values for the Adson test (positive predictive value PPV , 85%), the hyperabduction test (PPV, 92%), and the Wright test. Using several tests in combination improved specificity. Doppler ultrasonography visualized vascular parietal abnormalities and confirmed the diagnosis in patients with at least five positive provocative tests. Electrophysiological studies were useful mainly for the differential diagnosis or for detecting concomitant abnormalities. Although helical CT angiography provided accurate information on the location and mechanism of vascular compression, the usefulness of this investigation for establishing the diagnosis of TOS and for obtaining pretherapeutic information remains unclear.