Background: In this study, the associations of seasons with blood calcium levels in children aged 1-10 have not been evaluated. Methods: In 2012-2014, whole blood samples were collected from 2,562 children and calcium concentrations were determined by flame atomic absorption spectrometry. The associations of seasons with calcium levels were analyzed by multivariable regression. Results: The mean value of calcium concentrations was 1.61 0.13 mmo1/1 and the overall deficiency was 29.3%. Overall, compared to those in winter, children in spring and summer had significant lower calcium concentrations that decreased by 1.2% (f = 0.012; 95% CI: 0.021, 0.002) and 1.4% = 0.014; 95% CI: 0.023, 0.005), respectively; and corresponding higher calcium deficiencies than those in spring, summer, and autumn with odds ratios (OR) were 1.93 (95% CI: 1.39, 2.66), 1.65 (95% CI: 1.21, 2.24), and 1.57 (95% CI: 1.14, 2.15), respectively. Moreover, this seasonality was more significant in girls in whom calcium concentration in summer decreased by 1.9% = 0.019; 95% CI: 0.036, 0.003) and OR for calcium deficiencies in summer was 2.46 (1.38-4.41), compared to the girls in winter. Conclusions: The seasons have significant association with blood calcium levels, especially in girls. However, the impact of this seasonality on children's health is still unknown. J. Clin. Lab. Anal. 30:741-744, 2016. (C) 2016 Wiley Periodicals, Inc.