To compare the efficacy of morning and evening latanoprost/timolol fixed-combination (LTFC) dosing in patients with primary open-angle glaucoma (POAG) and ocular hypertension.In this double-blind, randomized clinical trial, 63 untreated Chinese patients with POAG and ocular hypertension were enrolled. All patients received LTFC and were randomized (1:1) to group 1, morning (8 AM) dosing, or group 2, evening (8 PM) dosing. Vehicle drops were used in the morning or evening, accordingly, to preserve masking. Patients were treated for 4 weeks. Outcomes included mean reduction of the 24-hour intraocular pressure (IOP) and IOP fluctuation from baseline after a 4-week treatment.Fifty-six patients were included in the final analysis. In both groups, the posttreatment IOP values were significantly lower than those at baseline at each 24-hour measuring time point. A significant difference between the groups in IOP reduction from baseline was observed at the 9:30 AM time point (4.01 ± 2.62 vs. 2.42 ± 3.23 mm Hg, evening dosing versus morning dosing group; P = 0.048). Both groups showed decreased IOP fluctuation after treatment. However, the morning dosing group had a significantly greater decrease in diurnal IOP fluctuation than that of the evening dosing group (2.04 ± 2.32 mm Hg vs. 0.50 ± 1.70 mm Hg, respectively; P = 0.012).Both morning and evening LTFC dosing can effectively reduce 24-hour IOP and IOP fluctuation. Morning dosing is more likely to effectively control diurnal IOP fluctuations.This multicenter, double-blind, randomized clinical trial generates robust evidence on the optimal LTFC dosing regimen to help clinical decision-making in the treatment of raised IOP.