A multicenter, randomized, investigator-masked, parallel-group trial compared bimatoprost and latanoprost for efficacy and safety in patients with glaucoma or ocular hypertension. Patients received bimatoprost 0.03% (n = 119) or latanoprost 0.005% (n = 113) once daily in the evening for 3 months. Visits were at prestudy, baseline (day 0), week 1, and months 1, 2, and 3. Primary outcome measures were mean IOP and the percentage of patients achieving IOP of 17 mm Hg or lower at 8:00 AM. Secondary outcome measures were diurnal IOP measurements (8:00 AM, 12 noon, 4:00 PM, 8:00 PM) at month 3 and safety measures including adverse events. Mean IOP was lower with bimatoprost than with latanoprost at all time points during the 3-month follow-up, although the between-group difference was not always statistically significant. At month 3 at 12 noon, mean IOP was as much as 1.0 mm Hg lower with bimatoprost (P = .021). Target pressures of < or = 17 mm Hg were reached more often with bimatoprost than with latanoprost at 8:00 AM (53% vs 43%; P = .029). Over all diurnal measurements at month 3, low target pressures of < or = 13, < or = 14, and < or = 15 mm Hg were achieved significantly more often with bimatoprost (P < or = .006). Both drugs were safe and well tolerated. Conjunctival hyperemia was more common with bimatoprost, while headache was more frequent with latanoprost. Bimatoprost provided lower mean pressures than latanoprost at every time point throughout the study and was statistically superior in achieving low target pressures. More patients reached low target pressures with bimatoprost.