Regular physical activity is a very important factor in the healthy development of an individual and an essential part of a healthy lifestyle. However, today's population still suffers from insufficient amount of exercise caused mainly by technical progress and often inappropriate conditions for practicing sports. In relation to this, we are grappling with a steady increase in obesity. During the COVID-19 pandemic, conditions for regular physical activity became even more unfavourable, with the declaration of a state of emergency and anti-pandemic measures leading to the closure of sports grounds and sporting competitions. In our research survey, we attempted to determine whether there would be changes in physical activity and BMI before and after the situation caused by the COVID-19 disease in upper primary school pupils in the Czech Republic. Using a questionnaire survey of a sample of children (n =1456), we found that already before the pandemic, 69% of the observed sample had not met the recommended amount of physical activity and only 67% of the sample was of normal weight. After the end of the anti-pandemic measures, the observed values deteriorated even more. There was a significant decrease in children with normal weight (by 7.5%) and an increase in children with Class 1 obesity. At the same time, we saw a significant decrease in children doing sports.