Objective: Endoscopic thoracic sympathectomy is performed to treat primary hyperhidrosis. The second and third sympathetic thoracic ganglia excised also innervate the heart. Some studies have shown decreased heart rate but have not been conclusive regarding other cardiac effects of sympathectomy. We studied the cardiac autonomic effects of endoscopic thoracic sympathectomy in a group of patients with primary hyperhidrosis. Heart rate variability is a simple, noninvasive electrocardiographic marker reflecting the activity and balance of the sympathetic and vagal components of the autonomous nervous system. Methods: We performed a prospective study in 38 patients with primary hyperhidrosis with 24-hour Holter recordings obtained before endoscopic thoracic sympathectomy and 6 months later. Results: We found statistically significant differences (P < .05) in both time and frequency domains. Parameters that evaluate global cardiac autonomic activity (total power, SD of normal R-R intervals, SD of average normal RR intervals) and vagal activity (rhythm corresponding to percentage of normal R-R intervals with cycle greater than 50 ms relative to previous interval, square root of mean squared differences of successive normal R-R intervals, high-frequency power, high-frequency power in normalized units) were statistically significantly increased after sympathectomy. Low-frequency power in normalized units, reflecting sympathetic activity, was statistically significantly decreased after sympathectomy. Low-/high-frequency power ratio also showed a significant decrease, indicating relative decrease in sympathetic activity and increase in vagal activity. Conclusion: These results provide, for the first time to our knowledge, clear evidence of increased vagal and global cardiac autonomic activity and decreased sympathetic activity after endoscopic thoracic sympathectomy.