PURPOSE: To evaluate the probability of a single intraocular pressure measurem ent to be the highest measurement within a diurnal intraocular pressure profile. DESIGN: Hospital-based clinical, observational study. METHODS: The study inclu ded 3,025 day-and-night intraocular pressure profiles measured on 1,072 eyes o f 547 Caucasian glaucoma patients or glaucoma suspects. Applanation tonometrywas performed at 7 am, noon, 5 pm, 9 pm, and midnight. RESULTS: Intraocular pressure measurements were highest at 7 am, noon, 5 pm, 9 pm, and midnight, respectively, in 20.4%, 17.8%, 21.3%13.9 %, and 26.7%of the profiles, respectively. The measurement taken at 7 am was s ignificantly (P< .001) closest to the maximal value of the profile. CONCLUSIONS: Any single intraocular pressure measurement taken between 7 am and 9 pm has a h igher than 75%chance to miss the highest point of a diurnal curve. Intraocular pressure may be measured at different times of the day to have the best chance o f observing the maximal value. PURPOSE: To evaluate the probability of a single intraocular pressure measurem ent to be the highest measurement within a diurnal intraocular pressure profile. DESIGN: Hospital-based clinical, observational study. METHODS: The study inclu ded 3,025 day-and-night intraocular pressure profiles measured on 1,072 eyes of 547 Caucasian glaucoma patients or glaucoma suspects. Applanation tonometry was performed at 7 am, noon, 5 pm, 9 pm, and midnight. RESULTS: Intraocular pressure measurements were highest at 7 am, noon, 5 pm, 9 pm, and midnight, respectively, in 20.4%, 17.8%, 21.3% 13.9%, and 26.7% of the profiles, respectively. The measurement taken at 7 am was ignificantly (P <.001) closest to the maximal value of the profile. CONCLUSIONS: Any single intraocular pressure measurement between 7 am and 9 pm has ah igher than 75% chance to miss the highest point of a diurnal curve. Intraocular pressure may be measured at different times of the day to have the best chance of observing the maximal value.