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Objective: This study aimed to verify vitamin D concentration in children and adolescents during the seasons of the year and to compare vitamin D concentration between children engaged in outdoor activities and those engaged in indoor activities., Methods: This is a cross-sectional study with a sample of 708 children and adolescents (aged 6-18 years), excluding 109 (16 were over 19 years old; 39 had a disease that required continuous treatment; 20 were on continuous medication; and 34 had no vitamin D data), ending with 599. The plasma concentration of 25-hydroxyvitamin D2 was measured with commercial kits following manufacturer instructions., Results: Participants who engaged in outdoor activities, as well as those who had data collected during summer and spring, had higher levels of vitamin D. According to the Poisson regression, the proportion of participants with inadequate levels of vitamin D was greater in the participants whose vitamin D was measured during spring (PR 1.15, 95%CI 1.03-1.29) and winter (PR 1.18, 95%CI 1.05-1.32). Also, a greater proportion of inadequate vitamin D was observed for those engaged in indoor activities (PR 1.08, 95%CI 1.01-1.15)., Conclusions: Participants who measured the vitamin during the summer and autumn had a lower prevalence of hypovitaminosis D. Even in regions with high solar incidence throughout the year, vitamin D levels can vary significantly during the period's seasons.