Objective: This study aimed to investigate the relations between calreticulin (CRT) serum level and both disease activity and severity parameters in juvenile idiopathic arthritis (JIA). Material and Methods: In this study, 60 children with JIA and 50 age-and-sex-matched healthy subjects were enrolled. The assessment of the disease activity was done using juvenile arthritis disease activity score 27 (JADAS-27). The assessment of disease severity was done via gray-scale ultrasonography (US) and power Doppler US (PDUS). Enzyme-linked immunosorbent assay (ELISA) was used to assay the serum level of human CRT. Results: The mean serum CRT levels in JIA patients was 8.6±1.2 ng/mL and showed a highly significant increase (p=0.001) as compared to the mean serum levels in the controls (5.02±0.77 ng/mL). There were statistically significant positive correlations between the serum CRT levels and disease duration, tender joint count, swollen joint count, visual analog scale, erythrocyte sedimentation rate, JADAS-27, C-reactive protein, rheumatoid factor titer, and ultrasonographic grading for synovitis and neovascularization. Conclusion: Elevated serum CRT levels in JIA patients and its correlations with JIA disease activity and severity parameters signified that CRT might be used as a novel biomarker for disease activity and severity in JIA. Keywords: Juvenile idiopathic arthritis, calreticulin, activity score, synovitis