Introduction and Aim: Juvenile idiopathic arthritis is a chronic disease affecting children up to the age of 16. The most common rheumatic disease of childhood is characterized by inflammation of one or more joints for at least six weeks. It can seriously damage the locomotor system and even lead to disability. The aim of this paper is to determine whether there are differences in the body mass index and physical activity of children suffering from juvenile idiopathic arthritis treated at the Clinical Hospital Center of Rijeka; location Kantrida in a relationship with healthy peers attending »Kantrida« Primary School. Participants and Methods: The research was conducted on 32 people. The examined group is children (13 respondents) aged 8 to 14 who have been treated at the Pediatric Clinic of Clinical Hospital Center Rijeka. The selected group is specific for age, possibly elevated body mass index, and possibly reduced physical activity. The control group consists of children from the »Kantrida« Primary School (17 respondents). The research was conducted on two locations; Pediatric Clinic of Clinical Hospital Center Rijeka and »Kantrida« Primary School. There are two parts of this research. In the first part of the survey, respondents completed a questionnaire on the physical activity of children, consisting of 10 questions, and in the second part the examiner carried out measurements of body height and body mass of respondents. After the measurement, a body mass index (BMI) was calculated. Results: By comparing BMI values of JIA and control groups, there was no statistically significant difference (p = 0.101) between them. Comparison of total physical activity between JIA and control group was statistically significant (p = 0.002). The correlation test showed statistically significant (p = 0.01) negative moderate correlation (r = -0.46) between BMI values and total physical activity, meaning that less physical activity during the week affects BMI increase. Conclusion: There is no statistically significant difference in body mass index of examined and control group but children affected by JIA are less physically active compared to control group.