Juvenile idiopathic arthritis (JIA) is the most common chronic inflammatory arthritis in children. Mean platelet volume (MPV) is an indicator of platelet size and has been investigated as an inflammation marker in several diseases. This study was designed to investigate the MPV values in patients with JIA and healthy subjects, and determine the correlation between MPV, C-reactive protein (CRP), and erythrocyte sedimentation rate (ESR). The hospital records of a total of 40 children with JIA, diagnosed using the International League of Associations for Rheumatology (ILAR) criteria, and 40 healthy controls were enrolled into the study. White blood cell count (WBC), platelet count, CRP, ESR, and MPV levels were retrospectively recorded. Children with JIA had significantly higher MPV values (8.28±1.12 fL) compared to the control group (7.53±1.07 fL) (p=0.003). Significant correlations were found between MPV, ESR, and CRP in the JIA group (r=0.676, p<0.001 and r=0.430, p<0.006). In addition, a negative correlation was found between platelet count and MPV (r= -0.818, p<0.001) in JIA patients. There was no significant difference in WBC values between the patient and the control groups (p>0.05). Our results suggest that MPV levels may be a useful marker of inflammation and prognostic factor for atherosclerosis risk in pediatric JIA patients.