Levobunolol hydrochloride (0.5% and 1%) and timolol maleate (0.5%) are being compared in an ongoing, double-masked, randomized study of 141 patients with ocular hypertension or chronic open-angle glaucoma. Baseline intraocular pressure (IOP) in the three treatment groups ranged from 26 to 27 mm Hg. During the first 15 months of the study, the two drugs have not proved to be significantly different in ocular hypotensive efficacy, with overall mean IOP decreases of 6.8 to 7.6 mm Hg. In addition, the two concentrations of levobunolol have been equally effective in controlling IOP. Neither drug has been associated with any significant ocular side effects. Both drugs have produced significant decreases (five to ten beats per minute) in mean heart rate. The effect on mean blood pressure has been less pronounced: overall decreases have been less than 4 mm Hg for both systolic and diastolic blood pressure. The results of this ongoing study suggest that levobunolol is as effective and as safe as timolol for the long-term control of IOP.