Objective: To examine how much physical activity the children at the Obesity Unit at a hospital in the middle of Sweden, between the ages of 14-17, registered and to compare differences between boys and girls and differences in how the children perceive their physical activity compared to the physical activity that they registered in an activity diary. Methods: A quantitative cross-sectional study was performed. Fifty-three children enrolled at the Obesity Unit at a hospital in central Sweden were included in the study, of whom 27 were boys and 26 were girls. Relevant data for the purpose of the study was collected from the questionnaires filled out by the participants during their first visit to the Obesity Unit and from their activity diaries. The participants were categorized and divided into groups according to their physical activity level and imagined physical activity level, whereupon statistical analyses were performed. Results: A little bit more than 22% of the participants attained the amount of physical activity recommended in Sweden for that age category. No statistically significant difference was found between gender and physical activity, however, a significant difference was found between how the questionnaires were answered and gender, which showed that girls overestimated their physical activity. Most of the study participants believed they performed less physical activity than their peers which is also accurate with how much physical activity they actually performed. Conclusions: In the study conducted, few of the children enrolled at the Obesity Unit attained NNR's recommendations for physical activity, however more extensive studies are acquired to be able to generalise this on a larger population. There was no statistically significant difference regarding the practice of physical activity between the sexes, to investigate this further deeper studies are required. A significant difference was found in how girls and boys rated their own physical activity where girls rated their physical activity higher than their male peers. Further studies are required to examine the reasons why. Comprehension about this phenomena is important to optimise interactions with this patient group.