Introduction: Juvenile idiopathic arthritis (JIA) is one of the most common chronic diseases in pediatric population. In the course of their disease, patients with enthesitis-related arthritis (ERA), one of seven subtypes of JIA, often develop inflammatory involvement of axial skeleton. Therefore, performing an adequate diagnostic procedure in time is essential for early diagnosis, introduction of therapy and avoiding joint damage and functional disability. Aims of the study: To determine the value of Color Doppler in detection and monitoring of sacroiliitis in patients with jSpA/ERA in comparison with MRI. To investigate the prevalence of acute and/or chronic sacroiliitis in these patients and analyse the correlation between radiological and clinical and laboratory findings. Moreover, to further explore the necessity of intravenous contrast application for detection of SI joints acute inflammatory changes on MRI. Materials and methods: 85 consecutive patients were included in this study, 57 girls and 28 boys with mean age of 14.25 years. Color Doppler ultrasound (CDUS) of sacroiliac joints was performed for each patient by two investigators and RI index was calculated. SI joints were also evaluated with an MRI exam by the same two investigators using standardized protocol. Medical history for each patient, relevant clinical data, and laboratory findings were collected. Results: Out of 85 patients, SI joint vascularization was determined with CDUS in 15 right and 19 left SI joints respectively by investigator 1 and in 9 (right) and 11 (left) SI joints by investigator 2. An increased number of doppler flow signals (1 DFS, >1 DFS in <50% of analyzed area, >1 DFS in >50% of analyzed area) resulted in a decrease of RI index for both investigators (right: 0.72-0.63-0.58, left: 0.70-0.65-0.54 - investigator 1; right: 0.70-0.65, left 0.70-0.67 - investigator 2). After data condensation for both investigators on arithmetic mean, a statistically significant correlation was determined for CDUS findings of joint vascularization, grade of vascularization and RI index, and MRI findings of acute sacroiliitis as a gold standard (right: 0.26, 0.27, 0.24, left: 0.26, 0.24, 0.25, p<0.05). Correlation coefficient is not high but suggests a statistically significant link between CDUS and MRI findings. CDUS sensitivity in detection of acute sacroiliitis varies between 0.26 and 0.39 depending on investigator, specificity is higher than 0.80 for both investigators. CDUS interobserver agreement is modest (0.64 right and 0.47 left) but very high for MRI (0.91 right, 0.85 left). Between patients with normal MRI findings and those with acute and/or chronic sacroiliitis, statistically significant differences were present for these variables: positive family history for inflammatory disease, erythrocyte sedimentation rate, and JADAS-10 score (both higher in patients with sacroiliitis). An extremely high correlation was established for detection of acute sacroiliitis on MRI when using both unenhanced and contrast enhanced MRI sequences. Conclusion: CDUS can detect acute sacroiliitis, correlation with MRI findings is statistically significant, but has a low coefficient giving it a sensitivity of 39%. The best correlation is achieved when 3 or more doppler flow signals are detected with CDUS and RI index is <0.60. A high specificity of 80-95% makes it possible to use CDUS as a screening method. Administration of contrast agent is not necessary for detection of acute sacroiliitis on MRI.