PURPOSE. To describe the distribution of the 24-hour intraocular pressure (IOP) among subjects with primary open-angle glaucoma (POAG) in a population-based study in north China. METHODS. All untreated POAG patients (n = 66) identified in the Handan Eye Study were invited to attend a follow-up study in which IOP was tested with a Goldmann applanation tonometer at 10 AM, 2 PM, 6 PM, 10 PM, 2 AM, and 6 AM. RESULTS. Forty-seven subjects with untreated POAG (47/66; 71.2%) attended the study. Of them, 39 (83.0%) had a peak IOP <= 21 mm Hg. The peak IOP (mean +/- SD) was 18.4 +/- 3.7 mm Hg. Peak IOP occurred in early morning in approximately 76.5% of the subjects (6 AM to 10 AM), and the trough occurred during night time in 70.2% of the subjects (10 PM to 2 AM). The 24-hour IOP (mean +/- SD) was 15.4 +/- 3.1 mm Hg and the mean fluctuation was 6.0 +/- 2.2 mm Hg (range, 2-11 mm Hg). In the 22 persons with unilateral glaucoma, no significant differences were found in mean 24-hour IOP, peak IOP, trough IOP, or IOP fluctuation when comparing the glaucomatous eye with the nonglaucomatous eye (P > 0.05). CONCLUSIONS. About 80% of Chinese persons with POAG identified in a population-based study had maximum IOPs of 21 mm Hg or less over a 24-hour period. Twenty-four-hour IOP was similar between glaucomatous and contralateral nonglaucomatous eyes suggesting that factors other than IOP may play a role in the development of glaucomatous optic neuropathy in these eyes. (Invest Ophthalmol Vis Sci. 2011;52:7817-7821) DOI:10.1167/iovs.11-7528