Abstract. Background: Health behaviors are forms of human activity that have specific consequences to physical and mental wellbeing. Active forms of spending free time and optimal nutrition can be essential for biological, mental, and social development of a young person. A diet for children and adolescents should be varied and provide protein, carbohydrates, fats, vitamins, and bioelements. Aim: The aim of this study was to analyze how a diet and forms of spending free time are related to serum levels of bioelements in middle-school students. Material and methods: The study, which involved 376 middle-school students at the age of 13 – 16 years, consisted of two stages. The first stage was survey-based and conducted using an international standard research tool – the Health Behavior in School-aged Children: A WHO Collaborative Cross-national Study (HBSC). As the next stage, venous blood sample was collected and laboratory analysis was performed to determine the levels of bioelements (Mg, Ca, Cu, Fe, Zn) in blood serum. Results: Laboratory analysis did not demonstrate any deviations from the reference ranges for the levels of bioelements. Statistical analysis revealed statistically significant differences in Ca levels between diet users and non-users. There was also a statistically significant correlation between Zn levels and the number of hours per day spent by the middle-school students watching TV in their free time, both school-days and on weekends. Analysis of the research material demonstrated statistically significant correlation between Fe levels and hours per day that the middle-school students dedicated to using a computer in their free time, both on school-days and on weekends. Conclusions: 1. The influence of a diet and forms of spending free time on the levels of bioelements in the middle-school students has not been clearly determined. 2. The diet contributed to an increase in Ca levels in the adolescents analyzed in the study. 3. There were weak but statistically significant correlations between sedentary activities (watching TV, using a computer) and serum Zn and Fe levels in the middle-school students analyzed in the study.