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BACKGROUND: The central role of Vitamin D is the homeostasis of calcium and phosphorus, affecting bone metabolism directly. The principal source of Vitamin D is the synthesis by the skin in response to its exposure to ultraviolet B radiation. The synthesis of vitamin D by the skin is affected by different factors such as age, pigmentation, use of sunscreen, and clothing. The low sun exposure, especially in extreme latitudes, is associated with a more frequent prevalence of vitamin D deficiency. Objective: To determine if there are variances in 25-OH-VitaminD3 concentration throughout the season in school-aged children and their relationship with PTH, alkaline phosphatase (AP), and calcium, and its association with UVB radiation. Methods: Cross-sectional study of 5–8 years old children without vitamin D supplementation. All subjects were recruited in Santiago de Chile (latitude 32°55’ to 34°19’ south) during different seasons of the year, and underwent a complete physical examination, measures of height, weight, and BMI, and biochemical analysis, including 25-OH-VitaminD3 (by liquid chromatography and tandem mass spectrometry), PTH, AP, and calcium. UVB radiation measures obtained from the Chilean Meteorological Service’s database. Results: A total of one hundred thirty-three children were recruited (summer = 41, autumn = 28, winter = 35, spring = 29). No differences in sex, age, height-SDS, and BMI-SDS were found between the groups. 25-OH-VitaminD3 mean difference was significative comparing summer with winter (9.6 ng/mL, p <0.0001), autumn (6.9 ng/mL, p <0.001), and spring (5.4 ng/mL, p <0.01). No difference was observed in calcium concentration. AP and PTH mean difference also was significative comparing summer with winter (AP -47.5 IU/L, p <0.01; PTH -11.1 pg/mL, p <0.0001), autumn (AP -54.7 IU/L, p <0.01; PTH -8.3 pg/mL, p <0.001) and spring (AP -49.9 IU/L, p <0.05; PTH -10.8 pg/mL, p <0.0001). Vitamin D deficiency, insufficiency, and sufficiency status showed a seasonal variation (Pearson’s χ2 (6) = 36.6, p <0.001). Sufficiency percentage was higher in summer (51.2%) compared with autumn (10.7%, Odd ratio= 8.7, 95% CI= 2.5 to 30.0, p=0.0007) and winter (8.6%, Odd ratio= 11.2, 95% CI= 3.2 to 38.0, p<0.0001) 25-OH-VitaminD3 showed an inverse correlation with PTH concentration (r= -0.383, R2= 0.15, p <0.0001), and in turn, PTH was directly correlated to AP (r= 0.240, R2= 0.06, p= 0.006). 25-OH-VitaminD3 concentration was directly related to the UV index (r= 0.531, R2= 0.28, p <0.0001) Conclusion: 25-OH-VitaminD3 concentration decrease significatively during autumn and specially during winter, associated to an increase in PTH and AP increase, but not a change in calcium concentration. During the darker months, the reduction of UVB radiation seems to be related to the decrease of 25-OH-VitaminD3 concentration, increasing the percentage of Vitamin D deficiency and insufficiency status.