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To describe the prevalence and systemic associations of keratoconus in young adults in Perth, Western Australia. Cross-sectional study. One thousand two hundred fifty-nine participants 20 years of age. The Raine Study is a multigenerational, longitudinal cohort study based in Perth, Western Australia. This study represents a cross-sectional analysis of the birth cohort on returning for a 20-year follow-up. Participants underwent a detailed ophthalmic examination, including visual acuity assessment and Scheimpflug imaging using the Pentacam (Oculus, Wetzlar, Germany), and completed a health questionnaire. Keratoconus was defined as a Belin/Ambrόsio enhanced ectasia display score of 2.6 or more in either eye based on Pentacam imaging. Prevalence of keratoconus in this cohort. Of the 1259 participants, 50.8% were women and 85.7% were White. Fifteen participants had keratoconus in at least 1 eye, giving a prevalence of 1.2% (95% confidence interval, 0.7%–1.9%), or 1 in 84. A significant difference was found in best-corrected visual acuity (0.01 logarithm of the minimum angle of resolution vs. –0.05 logarithm of the minimum angle of resolution; P = 0.007), cylinder (1.25 diopters [D] vs. 0.25 D cylinder; P < 0.001) and spherical equivalent (–1.42 D vs. –0.50 D sphere; P = 0.02) on objective refraction, mean keratometry of the steep meridian (45.19 D vs. 43.76 D; P < 0.001), and mean corneal thickness at the thinnest point (475 μm vs. 536 μm; P < 0.001) between those with and without keratoconus. Keratoconus was associated with regular cigarette smoking (38.5% vs. 14.6%; P = 0.04), but showed no association with gender, race, body mass index, use of spectacles or contact lenses, history of allergic eye disease, or pregnancy. The prevalence of keratoconus in this Australian population-based study of 20-year-old adults was 1.2% (95% confidence interval, 0.7%–1.9%), or 1 in 84, which is one of the highest reported in the world. This has important implications for screening individuals at a younger age so that treatment can be initiated before disease progression. [ABSTRACT FROM AUTHOR]