OBJECTIVE To evaluate the 24-hour efficacy and safety of the latanoprost-timolol maleate-fixed combination vs latanoprost therapy in patients with primary open-angle glaucoma. METHODS A prospective, observer-masked, crossover, active-controlled, randomized comparison in which after a 6-week medicine-free period, patients were randomized to either latanoprost-timolol-fixed combination therapy or latanoprost therapy, both dosed once each evening, alone for 8 weeks. Patients were then switched to the opposite treatment for 8 weeks. At the end of the washout and treatment periods, a 24-hour diurnal curve was performed. RESULTS The baseline untreated mean +/- SD diurnal curve in 37 patients who completed the study was 24.2 +/- 2.0 mm Hg. The mean diurnal curve was 19.2 +/- 2.6 mm Hg for those who received latanoprost therapy alone and 16.7 +/- 2.1 mm Hg for those who received the fixed combination therapy (P<.001). The fixed combination therapy also provided a lower absolute intraocular pressure level (1.5-2.9 mm Hg, P<.001) and a greater intraocular pressure reduction from the untreated baseline (P<.001). Stinging was statistically lower with latanoprost therapy alone (P = .04), but itching was statistically increased compared with the fixed combination therapy (P = .04). CONCLUSION The result of this study suggests that the latanoprost-timolol-fixed combination compared with latanoprost therapy alone provides improved intraocular pressure reduction over the 24-hour diurnal curve and for each individual time point in patients with primary open-angle glaucoma.