The aim of the present study was to evaluate different aspects of a device designed to monitor physical activity. Measurements of different axes and placement of the monitor were tested using a treadmill with ranging increments in incline or speed. The monitor was also used to assess the level of physical activity among children and adolescents with congenital heart disease and in healthy controls at the same age. The results indicate that the monitor is a valid and reproducible instrument for measurements of physical activity. The study revealed that the level of activity was higher for healthy boys than healthy girls (p<0.0001). Boys with congenital heart disease also displayed higher values compared to girls with congenital heart disease, although the difference was not significant (p=0.067). Healthy boys revealed a significantly higher level of activity than did boys with congenital heart disease (p=0.003), but no such difference was found in girls (p=0.757). Nor were any differences found between younger and older individuals among patients with congenital heart disease. Young healthy controls, however, showed significantly higher levels of activity than their older counterparts. There were differences in activity monitored during the week, with lower activity in the weekends, but the activity on the same day in different weeks seemed stable. Neither were there any differences between measurements over whole weeks. The results indicate that the Computer Science & Application monitor is a valid instrument for assessing physical activity. The monitor may also be used, therefore, to validate the levels of physical activity level in children with congenital heart disease after medical and surgical treatment.