Purpose: Patients with juvenile idiopathic arthritis (JIA) are less physically active than healthy peers. Therefore we developed an internet-based intervention to improve physical activity (PA). The aim of the study was to examine the effectiveness of the program in improving PA. Relevance: Evidence is accumulating that PA is effective in the primary and secondary prevention of several chronic conditions. PA is also associated with a reduction in all-cause mortality. There is evidence that in youth aged 6-16 years, PA controls body weight, reduces blood pressure in hypertensive youth, improves aerobic capacity, muscular strength, endurance and skeletal health, reduces anxiety and depression and improves self concept. PA also has a positive effect on academic performance, concentration, memory and classroom behavior. For children with JIA and other chronic diseases it is recognized that they could take advantage of the same health benefits. Participants: All patients with JIA, registered in the University Medical Center Groningen, the Netherlands and aged 8-12 years, were selected for this study. Inclusion criteria were a diagnosis of JIA, a good comprehension of the Dutch language and the availability of a home-based computer with an internet connection. Methods: PA was determined by activity-related energy expenditure (AEE), PA level, time spent on moderate to vigorous PA and the number of days with 1 hour or more of moderate to vigorous PAand was assessed with a 7-day activity diary. Aerobic exercise capacity was assessed by means of a Bruce treadmill test and recorded as maximum endurance time. Disease activity was assessed by using the JIA core set. Adherence was electronically monitored. Analysis: Descriptive statistics were used for patient's characteristics. The Kolmogorov-Smirnov test was used to test variables for normality of distribution. Paired samples t-test was used for within group differences. Results: Out of 59 patients 33 eligible patients were included and randomized in an intervention (n = 17, mean±SD age 10.6±1.5 years) and control waiting list group (n = 16, mean±SD age 10.8±1.4 years). All patients completed baseline and t = 1 testing. PA significantly improved in both groups. Maximum endurance time significantly improved in the intervention but not in the control group. In a subgroup analysis for patients with low PA (intervention n = 7 and control n = 5), PA improved in the intervention but not in the control group. The intervention was safe, feasible and showed a good adherence. Conclusions: An internet-based program for children with JIA, aged 8-12 years, directed at promoting PA in daily life, effectively improves PA in those patients with low PA levels. It can also improve endurance while it is safe, feasible and has a good adherence. Implications: PA in JIA can be improved through internet based interventions. Similar programs can be developed for other chronic conditions.