OBJECTIVE To evaluate the effect of bimatoprost/timolol maleate fixed combination (BTFC), latanoprost/timolol maleate fixed combination (LTFC), and travoprost/timolol maleate fixed combination (TTFC) on 24-h intraocular pressure (IOP) in patients with open-angle glaucoma. METHODS This prospective, observer-masked, randomized study included 50 patients with primary open-angle glaucoma. All patients were using hypotensive lipids and timolol maleate fixed combination treatment for ≥4 weeks and had an IOP ≤ 21 mmHg. Group 1 (n = 18) received BTFC, group 2 (n = 14) received LTFC, and group 3 (n = 18) received TTFC. All patients were hospitalized, and IOP was monitored for 24-h (10:00, 14:00, 18:00, 22:00, 02:00, and 06:00). Mean diurnal IOP variation measurements were taken between 06:00 and 18:00, and mean nocturnal IOP variation measurements were taken between 22:00 and 02:00. Mean IOP and IOP variation in the three groups were compared. RESULTS Mean 24-h IOP did not differ significantly between the three groups (group 1: 14.6 ± 2.9 mmHg; group 2: 14.1 ± 3.7 mmHg and group 3: 15.8 ± 2.0 mmHg; P > 0.05). Mean diurnal IOP variation was 4.6 ± 2.3 mmHg in group 1, 5.8 ± 2.4 mmHg in group 2, and 4.3 ± 1.7 mmHg in group 3, and mean nocturnal IOP variation was 3.2 ± 2.8 mmHg in group 1, 2.9 ± 1.9 mmHg in group 2, and 3.0 ± 1.6 mmHg group 3. There were not any significant differences in diurnal or nocturnal IOP variation between the three groups (P < 0.05). CONCLUSIONS All three fixed combinations effectively controlled IOP for 24-h and had a similar effect on diurnal and nocturnal IOP variations.