A major focus of our study was to determine the value of postoperative intraocular pressure (IOP) in predicting the outcome of trabeculectomy (TE). The medical charts of 547 patients undergoing glaucoma filtering surgery at the Department of Ophthalmology of the University of Cologne from 1987 to 1996 were reviewed. The status of the visual field, level of visual acuity, appearance of the bleb, cup/disc ratio and IOP were studied. Pre- and post-operative glaucoma medication was recorded. The eyes with congenital glaucoma and those treated with antimetabolites were excluded. The results are presented with particular emphasis being placed not only on intraocular pressure (IOP) control but also on the progression of glaucomatous damage (deterioration of visual field or disc damage) and the decrease of visual acuity. The tonometric success rate of TE in controlling the IOP < 21 mmHg was 61%. Defining the rigid criteria for success of trabeculectomy as an IOP < 21 mmHg, no further visual field loss, no disc damage and no additionally required surgical intervention due to glaucoma, the success rate decreased to 44%. The results indicate that other factors than normalization of IOP determine the success rate of TE. Should trabeculectomy be the therapy of first choice in the early stage of glaucoma? Should trabeculectomy fail to control the IOP in the first eye, would this allow options, such as the use of antimetabolites in the second eye?