In a long-term study of 252 eyes that had initial glaucoma filtration surgery, there was an overall success rate of 85%. Rigid criteria for success included an intraocular pressure equal to or less than 19 mmHg, no further visual field loss or disc damage, and no glaucomatous etiology for a decrease in visual acuity. The success rate was evaluated over an extended postoperative period with a range of 2 to 14 years and a mean follow-up of five years (61 months). Full thickness procedures had a higher success rate for a greater length of time, 88% (6 years) than did trabeculectomy, 76% (4 years). If an eye was considered successful at two years, the probability of success at five years was 94.5% for full thickness procedures and only 82% for trabeculectomies. Choroidal effusion associated with hypotony and shallow anterior chamber was the most frequent postoperative complication. There was no difference in the incidence between full thickness procedures and trabeculectomies. Cataract extraction was indicated more often following full thickness procedures, 34%, than trabeculectomies, 21%. However, the performance of a choroidal tap or subsequent cataract extraction did not appear to influence the success of the filtration operation. Bleb leaks and bleb infection occurred only after full thickness operations and were associated with a high 5/8 (62%) rate of bleb failure and loss of glaucoma control. Full thickness filtering operations appeared to insure a lower pressure from a longer period of time than does trabeculectomy.