In a three-month, double-masked, randomized clinical trial, we evaluated the once-daily ocular hypotensive efficacy of 0.25% levobunolol and 0.25% timolol in 80 patients with open-angle glaucoma or ocular hypertension. Thirty-seven of the 39 patients (95%) in the 0.25% levobunolol group and 35 of the 41 patients (85%) in the 0.25% timolol group successfully completed the three-month study period. The overall mean decrease in intraocular pressure was 5.3 mm Hg (22%) in the 0.25% levobunolol group and 5.4 mm Hg (22%) in the 0.25% timolol group. This difference was not statistically significant. In both treatment groups, effects on mean heart rate and blood pressure were minimal. The data suggest that levobunolol 0.25% and timolol 0.25%, administered once daily, are equally effective in the treatment of open-angle glaucoma and ocular hypertension.