Objective:To characterize diurnal intraocular pressure (IOP)-changes in primary open-angle glaucoma by reproducing IOPs based on patient posture. Methods: In 148 patients with untreated primary open-angle glaucoma who had IOPs recorded during clinic hours that were less than 21 mm Hg (average,14.8± 3.2 mm Hg),we measured IOP by noncontact tonometry every 2 hours from 6 AM to midnight and every 3 hours from midnight to 6 AM with patients sitting and supine. The IOP was reproduced by designating the sitting IOP as measurements taken when the patient was awake and the supine IOP as measurements taken when the patient was asleep for each individual. The reproduced diurnal IOP was composed of 12 measurements that included 2 to 4 IOP levels measured with the patients supine and the rest while they were sitting. Results: The peak of sitting diurnal IOP (mean± SD) for 148 patients was 16.0± 2.7 mm Hg,which was significantly lower than the peak of supine IOP (18.9± 3.9 mm Hg) or the reproduced IOP (17.5± 3.6 mm Hg) (P < .001 for both comparisons). The average reproduced IOP at each measurement time peaked at 3 AM during sleep; with sitting diurnal IOP or supine diurnal IOP,the peak IOPs were at noon. Twenty-nine patients (20% ) with an IOP less than 21mmHg during clinic hours had a reproduced IOP of 21mmHg or greater while asleep,compared with only 5 patients (3% ) when the patients were sitting only. Conclusions: In patients with primary open-angle glaucoma and IOPs less than 21 mm Hg during clinic hours,20% of patients had a reproduced IOP of 21 mm Hg or greater,compared with only 3% who had an IOP of 21 mm Hg or greater while sitting. Intraocular pressures peaked in most patients during sleep. Objective: To characterize diurnal intraocular pressure (IOP) -changes in primary open-angle glaucoma by reproducing IOPs based on patient posture. Methods: In 148 patients with untreated primary open-angle glaucoma who had IOPs recorded during clinic hours that were less than 21 mm Hg (average, 14.8 ± 3.2 mm Hg), we measured IOP by noncontact tonometry every 2 hours from 6 AM to midnight and every 3 hours from midnight to 6 AM with patients sitting and supine. The IOP was reproduced by designating the sitting IOP as measured taken when the patient was awake and the supine IOP as measurements taken the the patient was asleep for each individual. The reproduced diurnal IOP was composed of 12 measurements that included 2 to 4 IOP levels measured with the patients supine and the rest while they were sitting. Results: The peak of sitting diurnal IOP (mean ± SD) for 148 patients was 16.0 ± 2.7 mm Hg, which was significantly lower than the peak of supine IOP (18.9 ± 3.9 mm Hg) or the reproductive ced IOP (17.5 ± 3.6 mm Hg) (P <.001 for both comparisons). The average reproduced IOP at each measurement time peaked at 3 AM during sleep; with sitting diurnal IOP or supine diurnal IOP, the peak IOPs were at noon. Twenty-nine patients (20%) with an IOP less than 21 mmHg during the clinic hours had a reproduced IOP of 21 mmHg or greater while asleep, compared with only 5 patients (3%) when the patients were sitting only. Conclusions: In patients with primary open-angle glaucoma and IOPs less than 21 mm Hg during clinic hours, 20% of patients had a reproduced IOP of 21 mm Hg or greater, compared with only 3% who had an IOP of 21 mm Hg or greater while sitting. Intraocular pressures peaked in most patients during sleep.