The proposed study will compare for the first time the quality of 24-hour IOP control with Brimonidine/Timolol fixed combination (BTFC) versus Dorzolamide/Timolol fixed combination (DTFC) after a run-in period of 2 months with timolol. This crossover comparison may determine the real efficacy of the two fixed combinations and the design of the proposed study may explain for the first time why the 24-hour IOP reduction provided by BTFC is less than might be anticipated.

Inclusion Criteria:

• Consecutive, newly diagnosed or previously untreated POAG patients
• Patients with typical glaucomatous disc, or visual field damage
• Patient must have IOP (at 10:00 ± 1 hour)greater than 25 mm Hg

Exclusion Criteria:

• Uncontrolled glaucoma
• Distance best corrected Snellen visual acuity worse than 1/10
• Contraindications to brimonidine or dorzolamide and β-blockers
• History of lack of response (<10% morning IOP reduction) to any medication
• Patient can not understand the instructions and adhere to medications
• Patient is a female of childbearing potential or lactating mother
• Prior surgery, past use of steroids (within 2 months)
• Severe dry eyes and use of contact lenses
• History of non-adherence
• Patients with closed angles