The change in intraocular pressure achieved after 98 trabeculectomy operations performed on 70 patients with primary open-angle glaucoma was analysed. The reduction was proportional to the untreated preoperative pressure, and the results indicated that a first trabeculectomy reduced the intraocular pressure to between 16 and 20 mmHg irrespective of its initial level. Cases which required medical therapy for final pressure control after surgery showed a distribution of initial intraocular pressure similar to those not requiring such therapy. In addition, these cases were reduced to a level of pressure only slightly above the arbitrary figure of 20 mmHg before medical therapy was added, and were therefore considered almost to have reached the normal physiological range. Cases submitted to a second trabeculectomy are discussed, including 2 cases with unexplained acute open-angle glaucoma some months after the first operation.