Objectives: The aim of this study was to determine the trend of changes in body mass index (BMI) in primary school children in the city of Split over an 8-year period, considering factors such as involvement in sports, gender, and the educational level of their parents. Materials and Methods: The study was conducted through a retrospective analysis of medical records in school medicine clinics of the Public Health Institute of Split-Dalmatia County. The subjects were children from the city of Split who underwent systematic medical examinations by school medicine physicians in the 1st, 5th, and 8th grades of primary school. In this study, we performed a longitudinal analysis of students who enrolled in primary school in the 2016/2017 school year. Results: The study included 427 children who enrolled in the first grade of primary school in the city of Split during the 2016/2017 school year. In the 5th and 8th grades, boys who participated in sports exhibited a significantly lower BMI (P<0,001) compared to boys who did not participate in sports. Among girls, there were no statistically significant differences in BMI based on sports participation over the observed period. Children whose parents had completed higher education were more frequently involved in sports during all three systematic examinations compared to children of parents with secondary education (P<0,05). The prevalence of overweight students slightly increased with age. In the first grade, 14.05% of students were overweight or obese, in the fifth grade, it was 16.86%, and in the eighth grade, it was 16.16%. In the observed population, there were no significant differences in the prevalence of overweight and obesity between girls and boys. As children aged, the prevalence of overweight and obesity increased among children of mothers with secondary education compared to children of mothers with higher education. The same trend was observed for fathers. Conclusion: The prevalence of overweight and obesity increases with the age of the children. Boys who participate in sports have lower BMI values. Children of parents with higher education are more frequently involved in sports and have a lower prevalence of overweight and obesity compared to children of parents with secondary education.