<p>While serum ferritin has been established as a marker of disease activity in adults with rheumatoid arthritis, evidence on its role in children with Juvenile Idiopathic Arthritis (JIA) remains limited—particularly among non-systemic subtypes. We aimed to investigate the potential usefulness of serum ferritin levels as a marker of disease activity across the full spectrum of JIA subtypes in children. We retrospectively evaluated 139 children with JIA classified according to the International League of Association for Rheumatology (ILAR) criteria attending our Rheumatology Clinic. A comprehensive assessment was performed. Patients were classified by ILAR subtypes, and disease activity was assessed using JADAS-10 with standard cut-offs. Increased serum ferritin levels were observed in Still’s disease (sJIA) and rheumatoid factor (RF)-positive polyarthritis patients compared to other subtypes (<i>p</i> &lt; 0.0001). The highest serum ferritin levels were demonstrated in sJIA and RF-positive polyarthritis patients with high disease activity (<i>p</i> = 0.003). A positive correlation was found between serum ferritin and JADAS-10 score in the study population (r² = 0.37, adjusted r² = 0.36, <i>p</i> &lt; 0.0001).This association remained significant within the sJIA (<i>p</i> &lt; 0.0001) and RF-positive polyarthritis (<i>p</i> = 0.008) subtypes.</p><p><i>Conclusion</i>: Increased baseline serum ferritin levels were found in children with sJIA and RF-positive polyarthritis exhibiting high disease activity, compared to those with other JIA subtypes. These preliminary findings suggest serum ferritin may as a valuable marker of disease activity across JIA subtypes in pediatric clinical practice. Further prospective studies are warranted to confirm its usefulness in monitoring disease course and treatment response.<Table Float="No" ID="Taba"> <tgroup cols="4"> <colspec align="left" colname="c1" colnum="1" /> <colspec align="left" colname="c2" colnum="2" /> <colspec align="left" colname="c3" colnum="3" /> <colspec align="left" colname="c4" colnum="4" /> <tbody> <row> <entry nameend="c4" namest="c1"> <p><b>What is known:</b></p> <p><i>• Serum ferritin has been recognized as a marker of disease activity in adult patients with RA.</i></p> <p><i>• Evidence on the role of ferritin as disease activity marker in children with JIA remains limited, particularly for non-systemic forms.</i></p> <p><i>• Limited data exist on TPHA prevalence.</i></p> <p><b>What is new:</b></p> <p><i>• Serum ferritin levels are associated with high disease activity in children with sJIA and RF positive polyarthritis.</i></p> <p><i>• Serum ferritin may serve as a useful marker of disease activity across JIA subtypes, particularly in sJIA and RF-positive polyarthritis, potentially improving the challenging management of these high-risk patients.</i></p> </entry> </row> </tbody> </tgroup> </Table></p>