AIM:To evaluate the effect and safety of morning vs evening once daily of fixed combinations latanoprost/timolol maleate therapy in primary open angle glaucoma or in ocular hypertensive patients. METHODS:This study was prospective, observational clinial research. All ophthalmology examinations and 24 hours IOP testing were performed at the beginning of the study of latanoprost /timolol maleate.At 1 month follow-up, 24 hours IOP was tested and recorded the results and side effects. One week after stopping treatment, this was then changed to evening dosing once daily with fixed combinations of latanoprost/timolol maleate. After 1 month follow-up, 24 hours IOP was tested, the results and side effects were recorded. The IOP and side effects in two groups were then compared. RESULTS:Thirty-two patients completed this study. There was a significant reduction at each time point in the 24-hour diurnal curve of both morning (17.3±3.1mmHg) and evening (17.1±2.7mmHg) dosed patients, compared to the baseline IOP (21.1±3.3mmHg) (P<0.01). When the morning and evening dosing groups were compared directly, the 6∶00 time point was statistically lower with evening dosing (16.4±2.3mmHg) vs morning dosing (17.9±2.8mmHg) (P<0.05). A trend was observed, which indicated greater daytime reduction with night-time dosing, whereas morning dosing tended to give lower night-time pressure. There was a significantly lower mean range of diurnal pressure with evening (3.6±1.6mmHg) vs morning (4.4±1.6mmHg) dosing (P<0.05). There was no significant difference between two groups of side effects (P>0.05). CONCLUSION:This study suggests that fixed combinations latanoprost/timolol maleate both given once daily in the morning and evening can effectively reduce the IOP for the 24-hour diurnal curve and are well tolerated with few side effects. There is a significantly stable 24 hour IOP lowering in evening dosed fixed combinations of latanoprost /timolol maleate..