Purpose: To assess whether a change in myopia related oculometric parameters of primary school children in Beijing was associated with indoors and outdoors activity. Methods: The longitudinal school-based study included school children who were examined in 2011 and who were re-examined in 2012. The children underwent a comprehensive eye examination including ocular biometry by optical low-coherence reflectometry and non-cycloplegic refractometry. Parents and children had a detailed interview including questions on time spent indoors and outdoors. Results: Out of 681 students examined at baseline, 643 (94.4%) returned for follow-up examination. Within the one-year period, mean time spent daily outdoors increased by 0.4 +/- 0.9 hours, mean axial length by 0.26 +/- 0.49 mm, the ratio of axial length divided by anterior corneal curvature (AL/CC) by 0.03 +/- 0.06, and myopic refractive error by -0.06 +/- 0.89 diopters. In multivariate analysis, elongation of axial length was significantly associated with less total time spent outdoors (P = 0.02; standardized coefficient beta -0.12) and more time spent indoors with studying (P = 0.007; beta: 0.14) after adjustment for maternal myopia (P = 0.02; beta: 0.12). An increase in AL/CC was significantly associated with less time spent outdoors (P = 0.01; beta: -0.12) after adjustment for paternal myopia (P = 0.003; beta: 0.15) and if region of habitation was excludedors for leisure (P = 0.006; beta: -0.13), with less total time spent outdoors (P = 0.04; beta: -0.10), or with more time spent i. An increase in myopic refractive error, after adjustment for age, was significantly associated with less time spent outdo ndoors with studying (P = 0.005; beta: 0.13). Conclusions: A change in oculometric parameters indicating an increase in myopia was significantly associated with less time spent outdoors and more time spent indoors in school children in Greater Beijing within a study period of one year. Our study provides additional information on the potentially helpful role of outdoors activity in the prevention of myopia. Public health care measures such as school agendas may potentially take it into account.