BACKGROUND In endoscopic thoracic sympathectomy (ETS), it is required to perform accurate cautery of the sympathetic trunk. Monitoring of palmar skin blood flow and temperature has been used to assess the efficacy of ETS. This study investigated whether Perfusion Index (PI) is useful in assessing palmar skin blood flow and temperature in ETS. METHODS We studied 5 patients (1 man, 4 women) with palmar hyperhidrosis who had undergone a total of 10 ETS procedures. We measured skin blood flow, temperature and PI during ETS and evaluated the results. RESULTS Significant correlations were found between increases in skin blood flow and PI after ETS in cases with the palmar skin temperature just before ETS of below 35 degrees C. CONCLUSIONS In these cases, we can substitute increases in PI with increases in skin blood flow during ETS.