Obesity is a serious problem in our society. An evaluation of obesity development performed in the second half of the previous century already indicated a long-term positive trend in terms of body weight increase in children and the youth, which still persists today. Paediatric obesity arises from a changed lifestyle of children, characterised by an important restriction of their spontaneous physical activity. A lack of physical activity is one of the most important causes of paediatric obesity, which associated with a number of serious disorders. In the current study, the incidence of obesity and overweight as well as the relationship between physical activity and obesity in second stage pupils of elementary schools in northwest Bohemia is presented. The data collection was based on questions from the NAS 2001 questionnaire (nationwide anthropological survey) and BMI-for-age. 2001. NAS 2001 is a questionnaire for children and evaluates areas focused on engagement in physical activities and other daily activities, eating habits, drinking regime and care of the body habitus. Problems with obesity and overweight are present, particularly in boys. Only a third of boys and girls engage in sufficient physical activity. Differences were shown in the study group, where groups with higher BMI values had lower values of physical activity. Subsequently, a relationship was shown between those who use their bicycle as a means of transport and spend their leisure time bicycling at the same time. More than two-thirds of the study subjects reported using a bicycle as a means of transport and using their bicycle in their leisure time as a means of being active; 93% of these subjects had normal body weight. Our results confirm the continued pandemic prevalence of obesity and indicate that appropriate physical activity should be included in the everyday life of children both at school and outside of school.