Introduction. According to the World Health Organization the number of children with an increased body weight is increasing every year. The impact on such a condition have: dietary mistakes, lack of free time to prepare appropriate meals by parents together with children and increasingly less interest of secondary school children in physical culture and physical activity. The aim. The main aim of the study is to show whether there are any differences in the body composition (body fat content) between girls and boys aged 11–12 and whether it depends on the gender. The detailed objective is to examine how the body weight varies among the 5th grade pupils in one of the primary schools in Radom according to their gender. Material and methods. The study group consisted of 100 students of the Primary School No. 3 in Radom. The sample group consisted of 48 boys and 52 girls aged 11–12. The body composition analyzer BC545 TANITA, which assesses values with the accuracy to 0,1 kg , was used when conducting a research. Apart from the body weight, the device evaluated the body fat content in the whole body as well as occurring only in torso. Based on the gathered data and after receiving the results of body measurements, the researchers calculated the BMI indicator. This allowed to state whether the child is underweight, normal weight, overweight or obese. In the case of boys, the BMI ranges are: 1st range – less than 15 means underweight, 2nd range where values are in the range of 15–20 is the normal weight, 3rd for the value of 20 – 23 indicates overweight, and the last 4th with BMI above 23 defines obesity. BMI values are differentiated by gender. This is due to the earlier anthropomorphic changes of the girls’ bodies. The BMI values for girls are as follows: 1st range – less than 14 means underweight, 2nd range for values 14–22 indicates normal weight, 3rd range 22–27 means overweight, and the last 4th where BMI is above 27 indicates obesity. Results. Based on the collected data, in the study group, there are not boys with BMI range less than 15. There are 26 boys in the normal weight (BMI between 15–20), 12 of the examined boys is overweight (BMI in the 20–23 range), and 10 of them has BMI over 23 that means that they are obese. In the case of girls, 3 of them are underweight with BMI under 14, 34 girls have normal weight (BMI in the 14–20 range), in 3rd BMI range 22–27 are 12 overweight girls, while only 3 of examined girls has obesity, that is BMI over 27. Conclusions. Based on the research, the following conclusions can be drawn. Our own research proves that there is a difference in body fat content in the human body depending on the gender. Based on the on the example of 5th grade pupils of the Primary School in Radom, larger fat content in the torso occurs in the case of boys while a total fat content is bigger in the case of girls. Own studies show that boys aged 11-12 tend to be more frequent overweight and obese than girls in the same age group. Overweight and obesity affects both girls and boys. Although, as the analysis shows, boys are more prone to not following the rules of healthy eating. Due to the constant increase of body weight, it is desirable to pay attention to eating habits and to implement regular examinations of school children. Parents, teachers, doctors and dietitians must educate the society and promote the physical activity as the main method of the fight against obesity and overweight.