Purpose To investigate factors associated with ocular axial elongation and myopia progression during a 4-year follow-up in primary school children in Beijing. Methods This school-based study included 382 grade-1 children at baseline in 2011 (age: 6.3 +/- 0.4 years) with 305 (79.8%) returning for the follow-up examination in 2015. At baseline and in yearly follow-up examinations, the children underwent a comprehensive eye examination including auto-refractometry, ocular biometry with measurement of axial length, and fundus photography. The parents underwent a standardized interview. Results During the study period, the mean axial length elongated by 1.15 +/- 0.56mm in boys and 1.10 +/- 0.63mm in girls. At baseline and at the end of follow-up, axial length was significantly (P<0.001) longer in boys, with no difference (P = 0.50) between genders in axial elongation. In multivariate analysis, greater axial elongation was associated (regression coefficient r(2):0.15) with less time spent outdoors (P = 0.004; standardized coefficient beta: -0.22), more time spent indoors with studying (P = 0.02; beta: 0.18) and paternal myopia (P = 0.03; beta: 0.16). Larger increases in the axial length/anterior corneal curvature (AL/CC) ratio were associated (r(2):0.09) with less time spent outdoors (PP = 0.003; beta: -0.22) and maternal myopia (PP = 0.02; beta: 0.18). Conclusions Myopic axial elongation during a 4-year follow-up was associated with shorter time spent outdoors and longer time spent indoors studying and with parental myopia. Other factors such as level of paternal education, family income, gender and region of habitation were significantly associated with axial elongation and with myopia progression only in univariate analysis.