AIMS: To evaluate the efficacy and safety of replacing current dual ocular hypotensive therapy with latanoprost 0.005% monotherapy in patients with open angle glaucoma. METHODS: This randomised, open label, parallel group, multinational study included 466 patients with open angle glaucoma currently on dual ocular hypotensive therapy, including a beta adrenergic receptor antagonist. Patients were assigned (1:3) to ongoing dual therapy or a switch to monotherapy with latanoprost 0.005% once daily for 6 months. Intraocular pressure (IOP) was measured at 10 am and 5 pm at baseline, month 3, and month 6. Groups were compared for differences in diurnal IOP change, IOP success rates (IOP < or =22 mm Hg with < or =15% increase from baseline), and clinical success rates (not requiring change in therapy). RESULTS: Baseline mean diurnal IOP was 17.8 (SD 2.0) mm Hg in the latanoprost group and 17.6 (2.1) mm Hg in the dual therapy group. After 6 months, mean diurnal IOP was reduced by 0.26 (0.18) (SEM 1.4%) mm Hg (p=0.153) in the group switched to latanoprost and by 0.37 (0.25) (2.1%) mm Hg (p=0.138) in those continuing dual therapy (difference: 0.11 mm Hg; p=0.641). Success rates defined by IOP criteria were 83% for latanoprost and 89% for continued dual therapy (difference: 6%; p=0.122). Clinical success rates were 97% for latanoprost and 99% for dual therapy (difference: 2%; p=0.161). Ocular adverse events were reported by 23% of patients in both treatment groups. CONCLUSION: Latanoprost monotherapy is a safe and effective alternative for many patients with open angle glaucoma requiring dual topical ocular hypotensive therapy for IOP control.