In a prospective randomized clinical trial we compared the efficacy and safety of trabeculectomy and thermosclerostomy for the treatment of open angle glaucoma. Forty-eight eyes was treated by trabeculectomy and 50 eyes were treated by thermosclerostomy. The mean follow-up period was 2.7 years. Thermosclerostomy lowered intraocular pressure about 3 mm Hg lower than trabeculectomy on the average. However, the percentage of eyes with pressures less than 22 mm Hg was not different for the two operations. The main difference between trabeculectomy and thermosclerostomy was the number of complications. Postoperative flat anterior chamber, decrease in visual acuity, cataract, hypotony, and thin bleb with late perforation or infection all occurred more frequently after thermosclerostomy. In conclusion, although trabeculectomy is slightly less effective than thermosclerostomy in lowering pressure, its relative freedom from complications make it is more desirable filtering operation.