Excessive and unhealthy diet, physical inactivity and genetic predisposition are some of the factors that can cause an increased body mass. The body mass index (BMI) is closely related to the amount of excessive fat tissue in the human body and when increased, can lead to cardiovascular diseases, diabetes and neoplasms. The aim of this paper is to show the movement of the nutritional status of both male and female children throughout their primary education in the Opatija area. The generation that has participated in this research has been enrolled in the first grade in 2010, in the fifth grade in 2014 and has completed primary education in 2017. The data were obtained from the school doctor who works at Institute for Public Health (County of Primorje Gorski Kotar-Office Branch Opatija) and was entitled to perform the medical check-ups for the primary school children. A total of 325 primary school children has participated in this research. The data used in this study included children's weight, height, age and sex. Due to a calculated BMI for every child, one of the categories of nutrition was determined: underweight, normal body weight, overweight and obesity. The results were compared with the guidelines of the following major organizations: Centers for Disease Control, World Health Organization and Prevention and International Obesity Task Force. The given results have indicated that obesity has been the most frequent among both male and female children and has had a tendency to decline in the fifth grade by comparison with the eight grade. Furthermore, it has been found that underweight has been more common among the female children in all three grades than it has been among the male children. In order to conduct a more comprehensive research it is necessary to include data about their eating habits, physical activity and socio-economical status.