Although twice-daily instillation of topical beta-blockers is the standard regimen for treatment of increased intraocular pressure, once-daily therapy might improve patient compliance and provide greater safety. In a three-month, double-masked clinical trial, 92 patients with open-angle glaucoma or ocular hypertension received levobunolol 0.5% or 1% or timolol 0.5% once daily, in both eyes. Overall mean decreases in intraocular pressure were significantly greater in the groups treated with levobunolol than in the group treated with timolol. Intraocular pressure decreases averaged 7.0 mm Hg with levobunolol 0.5%, 6.5 mm Hg with levobunolol 1%, and 4.5 mm Hg with timolol. The intraocular pressures of 72% (18 of 25 patients) of those treated with levobunolol 0.5%, 79% (22 of 28 patients) of those treated with levobunolol 1%, and 64% (16 of 25 patients) of those treated with timolol were successfully controlled during the study. Heart rate and blood pressure decreases were minimal with both levobunolol and timolol. Study results indicated that once-daily treatment with levobunolol and, to a lesser extent, timolol is sufficient to control intraocular pressure successfully and safely.