This open-label, monocular, two-phase trial was conducted to determine whether bimatoprost is effective in patients with open-angle glaucoma or ocular hypertension not responsive to latanoprost. After a 4-week washout of any ocular hypotensive agents, patients with IOP between 22 and 34 mm Hg (n=51) instilled latanoprost in one eye and were evaluated at weeks 4 and 8 (phase 1). Patients with an IOP reduction no more than 3 mm Hg at both visits were considered nonresponders to latanoprost and were switched to bimatoprost for 8 weeks (n=21) (phase 2). In the treated eyes, the mean reduction in IOP from baseline after 4 weeks of each medication was 2.3 mm Hg with latanoprost (P=.012) and 6.1 mm Hg with bimatoprost (P<.001). After 8 weeks, respective IOP reductions were 1.9 mm Hg (P=.027) and 5.4 mm Hg (P<.001). After the switch from latanoprost, 8 weeks of bimatoprost provided an additional mean IOP reduction of 3.5 mm Hg (P<.001).