IMPORTANCE: Myopia (nearsightedness) has its onset in childhood and affects about one-third of adults in the United States. Along with its high prevalence, myopia is expensive to correct and is associated with ocular diseases that include glaucoma and retinal detachment. OBJECTIVE: To determine the best set of predictors formyopia onset in school-aged children. DESIGN, SETTING, AND PARTICIPANTS: The Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) Study was an observational cohort study of ocular development andmyopia onset conducted at 5 clinical sites from September 1, 1989, through May 22, 2010. Data were collected from 4512 ethnically diverse, nonmyopic school-aged children from grades 1 through 8 (baseline grades 1 through 6) (ages 6 through 13 years [baseline, 6 through 11 years]). MAIN OUTCOMES AND MEASURES: We evaluated 13 candidate risk factors for their ability to predict the onset ofmyopia. Myopia onset was defined as -0.75 diopters or more ofmyopia in each principal meridian in the right eye as measured by cycloplegic autorefraction at any visit after baseline until grade 8 (age 13 years).We evaluated risk factors using odds ratios from discrete time survival analysis, the area under the curve, and cross validation. RESULTS: A total of 414 children becamemyopic from grades 2 through 8 (ages 7 through 13 years). Of the 13 factors evaluated, 10 were associated with the risk formyopia onset (P <.05). Of these 10 factors, 8 retained their association in multivariate models: spherical equivalent refractive error at baseline, parentalmyopia, axial length, corneal power, crystalline lens power, ratio of accommodative convergence to accommodation (AC/A ratio), horizontal/vertical astigmatism magnitude, and visual activity. A less hyperopic/moremyopic baseline refractive error was consistently associated with risk ofmyopia onset in multivariate models (odds ratios from 0.02 to 0.13, P <.001), while near work, time outdoors, and having myopic parents were not. Spherical equivalent refractive error was the single best predictive factor that performed as well as all 8 factors together, with an area under the curve (C statistic) ranging from 0.87 to 0.93 (95%CI, 0.79-0.99). CONCLUSIONS AND RELEVANCE: Futuremyopia can be predicted in a nonmyopic child using a simple, single measure of refractive error. Future trials for prevention ofmyopia should target the child with low hyperopia as the child at risk.