Purpose. The International Society of Geographical and Epidemiological Ophthalmology (ISGEO) suggests a case definition for the prevalence studies of glaucoma based on the 97.5th and 99.5th percentile of vertical optic cup distribution among the evaluated population. Although multiple studies evaluating the prevalence of glaucoma have been undertaken in the last 20 years, case definitions have varied, and data on the underlying population distribution are sparse. This study evaluates the population distribution of 97.5th and 99.5th percentile of vertical cup disc ratio (VCDR) and VCDR asymmetry (VCDRA) in the US population and its association with demographic characteristics, self-reported glaucoma, and visual field. Methods. The National Health and Nutrition Examination Survey (NHANES) is a nationally representative sample of the US population, which during the years 2005 to 2008 collected frequency doubling technology screening fields and digital fundus photography. Accounting for the complex design of the NHANES population, estimates of the 97.5th and 99.5th percentile of VCDR and VCDRA were calculated, and national estimates of glaucoma prevalence were generated. Associations between disc characteristics, demographic variables, and self-reported glaucoma were explored. Results. Approximately 2.11% (95% confidence interval, 1.55 to 2.67) of the US population older than 40 years has glaucoma based on ISGEO criteria. A much larger 5.13% (95% confidence interval, 4.43 to 5.85) of the US population older than 40 years self-reports having glaucoma. Based on the estimates from NHANES, 6.89% of the population has a VCDR or VCDRA >97.5th percentile in either eye or OU. For the at-risk population with VCDR/VCDRA above the 97.5th percentile, <20% reported having glaucoma, whereas for those at the 99.5th percentile, <50% reported having glaucoma. Conclusions. The prevalence of glaucoma from NHANES based on ISGEO criteria produces similar population estimated to other population-based studies. Self-reported glaucoma is ∼2.4 times more common than what the prevalence estimates suggest. Copyright © 2011 American Academy of Optometry.