In a double-masked, randomized, comparison titration study to determine the effective dose of topically applied levobunolol, three concentrations of levobunolol (0.25%, 0.5%, and 1%) and of timolol (0.125%, 0.25%, and 0.5%) were evaluated in patients with mild open-angle glaucoma or ocular hypertension. Following a washout of ocular hypotensive medication, twice-daily treatment in both eyes was initiated with the lowest of the three doses of either drug. The concentration was increased if intraocular pressure remained uncontrolled. Intraocular pressure was controlled in 63% (15 of 24) of the patients tested with the lowest concentration of levobunolol and 69% (18 of 26) with the lowest concentration of timolol. Overall, 75% (18 of 24) of the patients in the levobunolol group and 73% (19 of 26) of the patients in the timolol group had adequately controlled intraocular pressure. At the lowest concentrations tested, mean decreases from baseline in intraocular pressure ranged from 6 to 8 mm Hg in both treatment groups.