The complications of trabeculectomy were studied in two groups of patients taken from a stable white population. The first group who had had their operation when it was first introduced 22 years ago, had been previously treated with prolonged medication, the second group had been operated upon recently and had had short-term pre-operative medication. This study confirmed that trabeculectomy predictably reduces the intraocular pressure to within the accepted normal range and that the pressure level below which the intraocular pressure could not be expected to fall was congruent to 14 mm/Hg. It also revealed that although some post operative abnormality was noted in two-thirds of the patients there were no long term problems which could be related to any operative or immediately post-operative complications, including shallow anterior chambers, uveitis and hyphaema. However, there was a long term reduction in the visual acuity and visual fields of about one-third of the patients, which was not related to cataract formation, macular problems, the height of the preoperative intraocular pressure, the amount by which this fell as a result of the surgery, or the amount, length or type of preoperative medication given before the operation. Although there was some increase in cataracts throughout the whole long term group this was mainly in those who had cataract prior to surgery; the increase was not related to operations or any other factor other than corneo-lenticular contact post-operatively.