Background : Thoracoscopic sympathicotomy was thought to be a simple and safe method for hyper- hidrosis. There is a significant decrease in heart rate during the procedure, Also it has been suggested that the effect of the degree of sympathetic block on cardiac function was affected by the preoperative level of sympathetic activity. Our hypothesis was that the higher heart rate before sympathicotomy the more heart rate change during the procedure because heart rate is one of the indicators of sympathetic activity. Methods: Sixty two patients with essential hyperhidrosis undergoing thoracoscopic sympathicotomy were studied. During the procedure, we recorded blood pressure at left arm and heart rate after anesthetic induction as baseline values, immediately after left side sympathicotomy, and after right side sym- pathicotomy. Patients were divided by median value of preoperative heart rate (83 beats/min) into group I (n = 32), preoperative heart rate below 83 beats/min, and group II (n =30), above 83 beats/min. Results : The preoperative mean heart rates of group I and 1I were 72 ± 7.9 beats/min and 100 ± 12.5 beats/min respectively. After sympathicotomy, the decrease of heart rate compared to the pre- operative value in Group I was 6.6%, which was significantly lower than that of Group II, 17.6%. After left sympathicotomy, the number of cases of heart rate decreasing more than 10% was higher in Group II (20/30 cases) than Group I (13/32 cases). Conclusions: The main result of this study showed that the higher heart rate before sympathicotomy the more heart rate change during procedure. (Korean J Anesthesiol 2000; 39: 67~71)