Renewed interest has been shown in transthoracic endoscopic sympathectomy(TES) for the treatment of upper limb hyperhidrosia. TES has many advantages such as simplicity, minimal trauma, benign postoperative course and early ambulation. One-lung ventilation is mandatory to get the clear view of the anatomy of the sympathetic chain during TES. It is well known that hypoxic pulmonary vasoconstriction(HPV) plays an important role to protect hypoxemia during the atelectasis induced by one-lung ventilation. Thoracic sympathectomy may have effects on pulmonary vasculature(HPV) and hemodynamics during one-lung anesthesia. We tried to investigate these effects in 20 patients undergoing TES. Hemodynamic effects were evaluated by measuring mean arterial yressure, heart rate and central venous pressure. And HPV was evaluated by arterial blood gas analysis. All of these parameters were checked 10 minutes after endotracheal intubation in supine position, lateral decubitus position, one-lung ventilation and l0 minutes and 20 minutes after thoracic sympathectomy. Mean arterial blood pressure was decreased from 81.9±2.89 to 73.2±2.49 mmHg after thoracic sympathectomy and heart rate was decreased from 104.4±3.12 to 88.2±2.31beats/min. Arterial oxygen tension was decressed from 570.5±17.9 to 521.4±23.2mmHg after position change, and decreased to 271.1±28.1 mmHg under one-lung ventilation, and finally decreased to 217.0±18.3 mmHg after thoracic sympathectomy. With the above results, we can conclude that patients for TES should be carefully observed during and after the procedure, and hypoxic pulmonary vasoconstriction may be impaired after TES.