In advanced glaucoma, medication alone no longer reduces intraocular pressure adequately, and the eye has field defects. Before 1980, some type of filtering surgery, such as trabeculectomy, was the usual method of intervention. Since then, laser trabeculoplasty has become a popular alternative. Sometimes the first intervention chosen succeeds in controlling pressure for many years; at other times, the success lasts only a few weeks or months. Because success is limited, some patients, over time, need to undergo a sequence of surgical interventions. Little is known about which sequence gives the best long-range outcome.