The aim of treatment in primary angle glaucoma should be to reduce the intraocular pressure throughout the 24 hours to a level where no damage is done to the optic nerve head, thus preventing any field defect. Open angle glaucoma as it is presently defined requires a significantly raised intraocular pressure to be associated with detectable changes in the optic nerve and a consequent visual field defect. The goal is therefore unattainable whilst this definition remains. The best that can be achieved is to reduce the intraocular pressure to such a level that no further disc damage occurs. Unfortunately, because there is no method of measuring the intraocular pressure over 24 hours, large diurnal rises of pressure may well remain undetected, producing irreparable damage to the disc. Furthermore, significant nerve fibre loss occurs before any field defect is detected even by the most sophisticated testing techniques and some patients develop field defects in spite of normal intraocular pressures. Treatment therefore is largely empirical, relying on the coarse assessment of visual fields, intermittent measurements of intraocular pressure and the clinical observation of the optic nerve head. Chronic open angle glaucoma is a disastrous, insidious disease destructive to the optic nerve head. It is difficult to diagnose before serious damage has occurred and it is difficult to monitor once it has been discovered. Medical therapy was successful in preventing field loss in one third of patients when the medication was given infrequently and in low dosage. Higher dosages were poorly tolerated and the drugs were often not taken by the patient. No advantage was obtained by increasing the strength and frequency of the medication because any improvement was transient.(ABSTRACT TRUNCATED AT 250 WORDS)