PURPOSE: To compare the intraocular pressure (IOP)lowering effect and safety of latanoprost 0.005% once daily with that of unoprostone 0.15% twice daily for patients with primary open,angle glaucoma or ocular hypertension. DESIGN: Randomized clinical trial. METHODS: In a prospective, 8-week, investigator-masked, parallel,group study conducted at numerous centers in the United States, 165 previously treated patients with IOP greater than or equal to 25 mm Hg in one or both eyes after washout were randomly assigned to receive either latanoprost 0.005% once daily in the evening or unoprostone 0. 15 % twice daily. Observations procedures were Goldmann applanation tonometry, best-corrected visual acuity, slit lamp biomicroscopy, and ophthalmoscopy. The main outcome measure was change in the mean of the lops measured at 8:00 AM, 12 noon, and 4:00 PM between baseline (before treatment) and after 8 weeks of treatment. RESULTS: The change in the mean +/- SD of the IOPs measured at 8:00 AM, 12 noon, and 4:00 PM was -7.2 +/- 3.2 mm Hg (28%) for latanoprost (25.3 +/- 2.8 mm Hg at baseline to 18.2 +/- 2.8 mm Hg at 8 weeks) and -3.9 +/- 2.6 min Hg (15%) for unoprostone (25.5 +/- 3.3 mm, Hg at baseline to 21.6 +/- 4.0 mm Hg; P less than or equal to .001. No serious adverse event related to either medication was reported. CONCLUSIONS: Over an 8-week period, latanoprost 0.005% once daily lowered IOP more than unoprostone 0.15% twice daily in patients with elevated IOP. Both agents were safe and well tolerated.