Purpose: To assess the efficacy and safety of switching patients from bilateral latanoprost to bimatoprost in 1 eye while maintaining latanoprost in the fellow eye. PATIENTS AND Methods: This prospective, open-label, multicenter, uniocular (within-eye control) study was conducted from March 2005 to February 2007; 105 patients with glaucoma or ocular hypertension were enrolled. At baseline, patients using bilateral latanoprost were switched to bimatoprost treatment in 1 eye (study eye) and continued latanoprost treatment in the fellow eye (control eye). At 12 weeks, patients were offered bilateral bimatoprost for 12 additional weeks. Results: At week 12, the mean difference in intraocular pressure (IOP) from baseline was -3.0mm Hg in study eyes and -1.6mm Hg in control eyes, which equates to a further -1.4mm Hg (95% confidence limits: -1.9, -0.9) reduction in IOP in study eyes compared with control eyes (P<0.0001). Overall, 27% (28/104) more study eyes had -2.5mm Hg reduction in IOP than control eyes (P<0001). At week 24, the mean difference in IOP from baseline was -2.8mm Hg for study eyes and for control eyes that had switched to bimatoprost. Conjunctival hyperemia occurred more frequently and was more severe in bimatoprost-treated eyes at week 12 than at baseline (P<0.001). No patients withdrew from treatment because of conjunctival hyperemia. Conclusions: Clinically, these data suggest that switching patients from latanoprost to bimatoprost may result in further improvement in IOP control.