Compensatory hyperhidrosis (CH) is the most common side effect after a thoracoscopic sympathectomy. The fear of CH is the most common reason why patients do not undergo a sympathectomy, because it is an irreversible procedure unless removal clips are used. Unfortunately, clip removal for reversal of postsympathectomy CH has not been reliable. To address this issue, we developed a new technique of a temporary thoracoscopic sympathetic block that can hopefully predict if postsympathectomy CH is going to occur after sympathectomy for medical refractory primary hyperhidrosis (PH). We reviewed all patients who underwent a temporary thoracoscopic sympathetic block and subsequent thoracoscopic sympathectomy for medical refractory PH. All patients were concerned about the development of CH and requested the possibility of a reversible procedure. Twenty-five patients underwent a temporary thoracoscopic sympathetic block; 18 suffered from palmar, axillary, and plantar hyperhidrosis, 4 had axillary and plantar hyperhidrosis; and 3 had palmar and plantar hyperhidrosis. The sympathetic block and subsequent sympathectomy were performed as outpatient bilateral thoracoscopic procedures. Sympathetic blockade was performed at each level of the planned sympathectomy (T2, T3, and accessory nerves) with 2.5 cc 0.25% marcaine with epinephrine per level without complications. All patients had temporary relief of hyperhidrosis ranging from 1 to 10 days with a median of 4 days after the block. Three patients (12%) had temporary CH after the thoracoscopic block, 2 mild and 1 severe. All but 1 (4%), the severe CH patient, elected to proceed with the planned sympathectomy; all sympathectomy patients (100%) were cured of their excessive sweating. The 2 patients who experienced mild CH after the thoracoscopic block also had it after the sympathectomy. All patients were completely satisfied with the final results, even the 2 patients in whom mild CH developed. Temporary thoracoscopic sympathetic block is a reversible and accurate procedure for the determination of postsympathectomy CH. A temporary thoracoscopic sympathetic block followed by sympathectomy may be the best approach for the treatment of medically refractory PH in patients who are concerned about the development of postsympathectomy CH.