The primary objective of this crossover trial is to compare the 3-month mean 24-hour intraocular pressure (IOP) control and safety of dorzolamide/timolol fixed combination (DTFC) given twice daily, versus latanoprost/timolol fixed combination (LTFC) given in the evening and placebo given in the morning, versus adjunctive therapy with DTFC given twice daily and latanoprost 0.005% given once in the evening in open-angle glaucoma patients who are insufficiently controlled with latanoprost monotherapy.

Inclusion Criteria:

• Consecutive primary open-angle (POAG) and exfoliative glaucoma (XFG) patients will be recruited.
• Patients included will be older than 29 years
• Have early to moderate POAG, or XFG (less than 12 mean deviation visual field loss attributed to glaucoma and 0.8 or better vertical cup-to-disc ratio)
• Will be on therapy with latanoprost for more than 3 months;
• Have at treated baseline IOP at 10:00 (two consecutive readings) greater than 21 mm Hg
• Have a reliable visual field (at least two visual fields with less than 30% fixation losses, false positives or negatives)
• Have a best corrected distance Snellen visual acuity > 1/10
• Have corneal pachymetry within the 550 ± 55 μm range, understand the study instructions and are willing to attend all follow-up appointments
• Are willing to comply with study medication usage
• And have open, normal appearing angles

Exclusion Criteria:

• Patients will be excluded if they have: a risk for significant deterioration during the study
• Known previous history of lack of adequate response (< 10% reduction) to any topical glaucoma medication
• Less than 20% daytime IOP reduction on latanoprost;
• Systemic contraindications to topical beta-blockers (asthma, bradycardia, severe congestive heart disease)
• Known contraindications to prostaglandins, history of ocular herpetic disease, or cystoid macular edema
• History of trauma, inflammation, surgery or past use of steroids (within two months)
• Severe dry eyes
• Use of contact lenses
• Signs of ocular infection, except blepharitis
• Corneal abnormality that may affect IOP measurements
• Unwillingness to accept the risk for hyperchromia of the iris or development of hypertrichosis
• And females of childbearing potential or lactating mothers