Study aim: the aim of this study was to determine how body height, body weight, and moderate physical activity affected postural stability in young schoolchildren. Materials and methods: the study population consisted of 148 primary school pupils. To assess postural stability, we used the CQStab2P stabilometric equipment (the bi-platform version), and an assessment of physical activity was conducted via questionnaire. Results: the study revealed that both greater body weight and greater body height positively affected postural stability in the studied population. We found a low, yet significant correlation regarding postural stability in children who attended sports classes. In the cases of path length and mean amplitude of sway with eyes closed, the children who were more active in sports had better vertical postural stability. We did not find any relationships between static balance and the number of sports classes attended in a week, or with participating in extra academic classes. Conclusions: 1. Children’s postural stability improved with age. 2. Children who took part in sports classes had better body balance. 3. The number of sports classes attended did not correlate with better postural stability.