IMPORTANCE Myopia has reached epidemic levels among children in regions of East and Southeast Asia. Highmyopia is associated withmyopic macular degeneration, glaucoma, and retinal detachment. OBJECTIVE To determine the incidence ofmyopia and highmyopia based on refraction without cycloplegia among children in primary and junior high schools in China. DESIGN, SETTING, AND PARTICIPANTS This observational cohort studywas completed in Guangzhou, China. It consisted of a cohort from 19 primary schools, who were followed up from 2010 to 2015, and a cohort from 22 junior high schools, who were followed up from 2010 to 2012. All schools were randomly chosen at rates proportional to the number of schools in each of the city's 11 districts. Students with or withoutmyopia in grade 1 (primary school) or grade 7 (junior high school) were eligible for inclusion. Data analysis occurred from February 2017 to October 2017. MAIN OUTCOMES AND MEASURES Myopiawas defined as a spherical equivalent refraction (SER) of -0.50 diopters (D) or less, as measured by subjective refraction without cycloplegia; highmyopia was defined as a SER of -6.0 D or less. Annual incidences were defined as the proportion of participants each year found to havemyopia or highmyopia who did not previously have the condition. Height, weight, axial length (AL), corneal radius of curvature (CRC), and AL/CRC ratio were examined to assess if these measures were associated with futuremyopia or highmyopia. RESULTS A total of 4741 students with or withoutmyopia in either grade 1 for the primary school cohort (mean [SD] age 7.2 [0.4] years; 932 of 1975 [47.2%] female) or grade 7 for the junior high school cohort (mean [SD] age 13.2 [0.5] years; 1254 of 2670 [47.0%] female) were included. Baseline mean (SD) SER was 0.31 (0.86) D among 1975 students in grade 1 vs -1.60 (2.00) D among 2670 students in grade 7. Baseline prevalence ofmyopia was 12.0% in grade 1 students (n = 237 of 1969) and 67.4%in grade 7 students (n = 1795 of 2663). The incidence ofmyopia was 20% to 30% each year throughout both cohorts. The incidence of high myopia was initially less than 1% in the primary school cohort (grade 1: n = 2 of 1825; 0.1% [95%CI, 0.0%-0.3%]), but incidence exceeded 2%in the junior high school cohort (in grade 9: n = 48 of 2044; 2.3%[95%CI, 1.0%-3.7%]). CONCLUSIONS AND RELEVANCE The incidence ofmyopia among Chinese students based on refraction without cycloplegia is among the highest of any cultural or ethnic group. If confirmed with cycloplegic refraction, interventions to preventmyopia onset in Chinese populations should be initiated in primary schools.