Objective: To assess associations among outdoor activity, ocular biometric parameters, and myopia among grade 1 and grade 4 primary students in Beijing. Design: School-based, cross-sectional study. Participants: A total of 382 grade 1 and 299 grade 4 children participated in the study. Methods: The children underwent a comprehensive eye examination, including ocular biometry by optical low-coherence reflectometry and noncycloplegic refractometry. Parents and children participated in a detailed interview, including questions on time spent indoors and outdoors. Main Outcome Measures: Factors associated with myopia. Results: The study included 681 children, with 382 (56.1%) students from grade 1 (mean age, 6.3 +/- 0.5 years; range, 5-8 years) and 299 students from grade 4 (mean age, 9.4 +/- 0.7 years; range, 8-13 years); 370 students (54.3%) lived in the urban region. The mean daily time spent outdoors was 1.6 +/- 0.8 hours (range, 0.5-5.1 hours). In multivariate analysis, axial length was significantly associated with older age (P < 0.001; standardized beta coefficient, 0.28), taller body height (P = 0.001; beta, 0.18), maternal myopia (P = 0.03; beta, 0.09), and urban region of habitation (P < 0.001; beta, -0.21), or alternatively to the region of habitation, with less time spent outdoors (P = 0.001; beta, -0.16) and more time spent indoors studying (P = 0.02; beta, 0.10). The axial length-to-corneal curvature radius ratio was associated with older age, urban region of habitation, maternal and paternal myopia, and paternal level of education. Presence of myopia (defined as refractive error <=-1 diopters in the right eye) was associated with older age (P < 0.001; odds ratio [OR], 1.45; 95% confidence interval [CI], 1.24-1.69), maternal myopia (P < 0.001; OR, 2.99; 95% CI, 1.94-5.35), and urban region of habitation (P < 0.001; OR, 0.17; 95% CI, 0.11-0.26), or alternatively to the region of habitation, with less time spent outdoors (P < 0.001; OR, 0.32; 95% CI, 0.21-0.48) and more time spent indoors studying (P < 0.001; OR, 1.38; 95% CI, 1.09-1.75). Conclusions: Less outdoor activity, more indoor studying, older age, maternal myopia, and urban region of habitation were associated with longer ocular axial length and myopia in grade 1 and grade 4 primary school children in Greater Beijing. Remaining outdoors more (e. g., during school) may reduce the high prevalence of myopia in the young generation in Beijing. Financial Disclosure(s): The author(s) have no proprietary or commercial interest in any materials discussed in this article. Ophthalmology 2013;120:277-283 (C) 2013 by the American Academy of Ophthalmology.