A major focus of our study was the ability to predict the long-term success of filtering surgery. The results of glaucoma filtering surgery (trabeculectomy) as performed by the staff of the University Eye Hospital of Tübingen from 1988 through 1994 on 254 eyes of 214 patients are presented, with particular emphasis being placed not only on intraocular pressure (IOP) control but also the progression of glaucomatous damage (visual field loss or disc damage) and the etiology of visual acuity losses. There was an overall success rate of 64%. Rigid criteria for success included an IOP of less than 30 mmHg, no further visual field loss or disc damage, and no glaucomatous etiology for a decrease in visual acuity. A total of 35 eyes (13.8%) showed a loss of visual acuity after 6 months amounting to 2 or more Snellen lines, caused mainly by lens opacification, hypotony maculopathy, and "wipeout" (loss of the central visual field in the absence of another explanation). We found that there is a considerable risk for sudden loss of visual acuity after operation on older patients with small residual visual fields and severe hypotony on the 1st postoperative day. There was a 13% incidence of failure of blebs. A total of 34 eyes had an early IOP rise of more than 30 mmHg after surgery. A postoperative hypotony of less than 3 mmHg was more significantly seen in patients treated with 5-fluorouracil. Moreover, the results show that in some instances, a long-term use of topically applied glaucoma medication can adversely affect the results of fistulizing surgery.(ABSTRACT TRUNCATED AT 250 WORDS)