In a one-year, double-masked, randomized study, the ocular hypotensive efficacy of twice-daily treatment with 0.25% levobunolol hydrochloride or timolol maleate was evaluated in 78 patients with glaucoma or ocular hypertension (phase 1). If intraocular pressure (IOP) was not well controlled during the study, the concentration of medication was increased to 0.5%, and the patient was followed up for an additional three months (phase 2). During phase 1, the mean IOP was reduced by 4.6 mm Hg in the timolol treatment group and by 5.1 mm Hg in the levobunolol treatment group. Seventy-one percent (29/41) of the patients in the timolol treatment group and 70% (26/37) of the patients in the levobunolol treatment group successfully completed phase 1. Of those patients who required the higher concentration of medication, 89% (8/11) in the timolol treatment group and 75% (3/4) in the levobunolol treatment group successfully completed phase 2. Higher concentration, however, did not produce greater IOP reduction. No statistically or clinically significant differences between the groups were noted in any of the efficacy or safety variables evaluated.