The results of 300 consecutive glaucoma filtering procedures, which were performed in our clinic between January 1982 and September 1985, are described. The overall combined success rate (complete + qualified) was 66.5% (Heuer's classification). This is in good agreement with other glaucoma referral centers. The peak of the age distribution curve at the time of operation was between 70 and 75 years. Nevertheless, 30% of operations was performed on patients under 50 years of age. Relatively poor success rates were obtained in operations following prior unsuccessful filtering surgery (50.5%) or other surgery (47%), in patients under 50 years of age (61%) and in some types of secondary glaucoma. In aphakia/pseudophakia the success rate was 33%. The posterior capsule was intact only 13% of this category at the time of surgery, in 56% previously unsuccessful filtering surgery had been performed, and the age at operation was relatively young (53 yr). These factors have an unfavorable influence on the results of filtering surgery. The success rate in phakic primary open angle glaucoma was 85% and in phakic primary angle closure glaucoma 73%. The combination with a cataract extraction (in 28%) and a peripheral iridectomy, if this had not been performed previously, (in 31%) may have contributed to the relatively favorable surgical results in phakic primary angle closure glaucoma. Relatively high intraocular pressures were found in the early postoperative period; the peak of the pressure-curve was between 20 and 25 mm Hg, 10 mm Hg higher than at the end of follow-up. Postoperative progression of glaucomatous visual field defects was noted in 7%. The most disturbing complication was flat anterior chamber, causing or enhancing cataract formation (in 4 out of 8 phakic patients) and corneal dystrophy (in 2 out of 12 patients). A review of literature is given concerning methods of improving filtration, when the surgical prognosis is poor.