The efficacy and safety of bimatoprost, a member of a new class of pharmacological agents called prostamides, were compared with the efficacy and safety of timolol in patients with glaucoma or ocular hypertension. Pooled 6-month results from two ongoing, multicenter, randomized, double-masked, clinical trials were analyzed. Patients were randomized in a 2:2:1 ratio to treatment with bimatoprost 0.03% once a day ([QD] n = 474), bimatoprost 0.03% twice a day ([BID] n = 483), or timolol 0.5% BID (n = 241). Scheduled visits were at prestudy, baseline, week 2, week 6, month 3, and month 6. The primary outcome measure was in diurnal intraocular pressure ([IOP] 8 AM, 10 AM, 4 PM, 8 PM). Bimatoprost QD provided significantly greater mean IOP reductions from baseline than timolol at every time of the day and at each study visit (p </=.05). BID dosing of bimatoprost also provided significantly greater mean IOP reductions than timolol at most timepoints, but was not as effective as QD dosing. The IOP lowering provided by bimatoprost QD was sustained for 6 months. At month 6, the mean IOP reduction from baseline at 10 AM was 8.1 mm Hg (33%) with bimatoprost QD, 6.3 mm Hg (26%) with bimatoprost BID, and 5.6 mm Hg (23%) with timolol. Low target pressures were achieved by a significantly higher percentage of patients in the bimatoprost QD group than in the timolol group. At 10 AM (peak timolol effect) at month 6, IOP </= 17 mm Hg was achieved by 63.9% of bimatoprost QD patients, compared with 37.3% of timolol patients (p <.001). Bimatoprost was safe and well-tolerated, with few discontinuations due to adverse events. The most frequent side effect was trace-to-mild conjunctival hyperemia. Changes in iris pigmentation were reported in 1.1% of bimatoprost patients. There were no other significant findings in slit lamp examinations, ophthalmoscopy, visual acuity, or visual fields, and systemic safety parameters were also unaffected. Together these results indicate that bimatoprost QD is statistically and clinically superior to timolol in lowering IOP, and is safe and well-tolerated in patients with glaucoma or ocular hypertension.