Objective: To evaluate latanoprost versus bimatoprost given each evening over the 24- hour diurnal curve. Design: Double- masked, 2- center, crossover comparison. Participants: Forty- two of 44 patients with primary open- angle glaucoma (POAG) completed the study. Methods: Consecutive patients were not treated during a baseline 24- hour curve after a glaucoma medicine- free period. They then were randomized to either latanoprost or bimatoprost for a 7- week treatment period. Diurnal curve intraocular pressures (IOPs) were measured at treatment period end at 2 AM, 6 AM, 10 AM, 2 PM, 6 PM, and 10 PM. After the first treatment period, patients were changed to the opposite medicine without a medicine- free period. Diurnal curve measurements were performed again at the end of the second 7- week treatment period. Main Outcome Measure: The 24- hour diurnal IOP. Results: On the last day of treatment, mean 24- hour IOPs were 17.3± 2.8 mmHg for latanoprost and 16.7± 2.4 mmHg for bimatoprost (P=0.01). The 6 PM individual time point for IOP was statistically lower for bimatoprost after a Bonferroni correction (P=0.008). The largest IOP difference at any time point was 0.9 mmHg at 6 PM. The most common side effect was conjunctival hyperemia, which occurred less with latanoprost (n=6) than with bimatoprost (n= 15) (P=0.004). Two patients had their treatments discontinued while on bimatoprost, one due to conjunctival hyperemia and the other due to ocular intolerance. Conclusion: This study indicates that the 24- hour diurnal IOP is statistically lower in POAG with bimatoprost, compared with latanoprost, among patients who tolerated bimatoprost. However, the IOP difference between groups was small and may not be clinically meaningful. In contrast, conjunctival hyperemia seems statistically greater with bimatoprost. The exact clinical importance of conjunctival hyperemia, if any, needs to be clarified further. Objective: To evaluate latanoprost versus bimatoprost given each evening over the 24-hour diurnal curve. Design: Double-masked, 2-center, crossover comparison. Participants: Forty- two of 44 patients with primary open- angle glaucoma They then were randomized to either latanoprost or bimatoprost for a 7-week treatment period. Diurnal curve intraocular pressure (IOPs) were measured at the treatment period end at 2 AM, 6 AM, 10 AM, 2 PM, 6 PM, and 10 PM. After the first treatment period, patients were changed to the opposite medicine without a medicine- free period. Diurnal curve measurements were performed again Results: On the last day of treatment, mean 24-hour IOPs were 17.3 ± 2.8 mmHg for latanoprost and 16.7 ± 2.4 mmHg for bimatoprost (P = 0.01). The largest IOP difference at any time point was 0.9 mmHg at 6 PM. The most common side effect was conjunctival hyperemia , which occurred less with latanoprost (n = 6) than with bimatoprost (n = 15) (P = 0.004). Two patients had their treatments discontinued while on bimatoprost, one due to conjunctival hyperemia and the other due to ocular intolerance. Conclusion: This study indicates that the 24-hour diurnal IOP is statistically lower in POAG with bimatoprost, compared with latanoprost, among patients who tolerated bimatoprost. However, the IOP difference between groups was small and may not be clinically meaningful. In contrast, conjunctival hyperemia seems statistically greater with bimatoprost. The exact clinical importance of conjunctival hyperemia, if any, needs to be clarified further.