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.