In 25 patients with ocular hypertension and open angles timolol maleate lowered the intraocular pressure (IOP) of 71% of the 49 eyes treated, keeping it below 22 mm Hg during 4 years of therapy. However, the IOP tended to drift upward by 2 to 4 mm Hg per year, especially during the first and third follow-up periods (1 year and 2 years long respectively). Thus, patients should be examined every 3 months after an adequate response to therapy is achieved. In this series of patients there was a slight visual loss in about 38% of patients, but it was probably related to increasing age. Field loss occurred in four patients in spite of a good response to therapy. Increased cupping of the optic disc occurred in six patients, four of whom had had a pretreatment IOP of 30 mm Hg or greater. Timolol therapy was well tolerated by the patients, being virtually symptom-free. However, special care would be needed in treating patients with asthma, bradycardia or congestive heart failure.