PURPOSE: To examine the longitudinal association between age and intraocular pressure (IOP) in a large sample of Korean men and women. METHODS: We conducted a prospective cohort study of 274,064 young and middle-aged Korean adults with normal fundoscopic findings, following them from January 1, 2002, to February 28, 2010. Health exams were scheduled annually or biennially. At each visit, IOP was measured in both eyes using automated noncontact tonometers. The longitudinal change in IOP with age was evaluated using three-level mixed models for longitudinal paired-eye data, accounting for correlations between paired eyes and repeated measurements over time. RESULTS: In fully adjusted models, the average longitudinal change in IOP per 1-year increase in age was -0.065 mm Hg (95% confidence interval [CI] -0.068 to -0.063), with marked sex differences (P < 0.001). In men, the average annual IOP change was -0.093 mm Hg (95% CI -0.096 to -0.091) throughout follow-up. In women, the average annual IOP change was -0.006 mm Hg (95% CI -0.010 to -0.003), with a relatively flat association in the age range of 30 to 59 years and more marked annual decreases at younger and older ages. CONCLUSIONS: Intraocular pressure was inversely associated with age in a large cohort of Korean adults attending health-screening visits. For men, this inverse association was observed throughout the entire age range, while for women it was evident only in younger (<30 years of age) and older (>/=60 years of age) women, with no association in women aged 30 to 59. Further research is needed to better understand the underlying mechanisms and to reconsider cutoffs for defining high IOP by age and sex groups in Asian populations.