· Background: The purpose of the study was to compare the effects on intraocular pressure (IOP) of adding dorzolamide to timolol or of switching from timolol to latanoprost monotherapy in glaucoma patients inadequately controlled on timolol. · Methods: The study was designed as a 3-month randomised, open-label, multicentre study comprising 183 patients with primary open-angle glaucoma, capsular glaucoma with IOP above 22 mmHg on treatment with one or two ocular hypotensive drugs, or ocular hypertension with IOP above 27 mmHg. After a 2- to 4-week run-in period on timolol, 0.5% twice daily, the patients were randomised to treatment with either latanoprost, 0.005% once daily, or the combination of timolol, 0.5% twice daily, and dorzolamide, 2% twice daily. The mean diurnal IOP after 3 months of treatment was compared with baseline. · Results: Switching from timolol to latanoprost reduced mean diurnal IOP by 4.5±0.2 mmHg (mean±SEM, ANCOVA; 20%), and adding dorzolamide to timolol reduced mean diurnal by 4.4±0.2 mmHg (mean±SEM, ANCOVA; 20%). No serious side effects were observed with either treatment. · Conclusion: Latanoprost monotherapy can be an alternative to combined treatment with two aqueous flow suppressors in patients whose IOP is insufficiently controlled by timolol alone.