The aim of this study was to compare the results of simultaneous bilateral thoracodorsal sympathectomy in the prone position with those of anterolateral sympathectomy performed in two staged, separate procedures for the treatment of bilateral excessive sweating of the hands and axillae, and to describe our technique for bilateral, simultaneous thoracodorsal sympathectomy. From July 1995 to March 2001, 202 thoracodorsal sympathectomies were done in 101 patients for severe hyperhydrosis. There were 79 females (age range 20-46) and 22 males (age range 19-65). In 52 patients, anterolateral sympathectomies were performed in the supine position, using unilateral lung collapse, with both sides operated on in two separate, staged procedures. In 49 patients, bilateral sympathectomy was conducted during a single procedure, in the prone position, without using unilateral lung collapse. In comparing the results from these two methods, we concluded that simultaneous bilateral thoracodorsal posterior sympathectomy, has comparable safety, may improve outcome, decreases in half the number of hospital admissions, and produces a significant overall reduction in cost when compared with staged anterolateral sympathectomy for the treatment of severe hyperhydrosis.