Background: Juvenile idiopathic arthritis (JIA) is generally considered a clinical syndrome involving several disease subsets, with a number of inflammatory flows, leading to an eventual common pathway in which persistent synovial inflammation and associated damage to articular cartilage and underlying bone are present. Neoptrin is a reliable marker in the assessment of the rate of IFN-γ production. Levels of neoptrin increase in direct proportion with the level of interferon. Measurement of neopterin level is useful because of its relative stability also it is a prognostic indicator for cell-mediated immunity. Aims: This study aims to assess serum level of neopterin in patients with Juvenile Idiopathic Arthritis (JIA) in relation to the disease activity, severity and response to conventional and biological therapy. Methodology: The study was conducted on 30 patients (Group A) previously diagnosed as SoJIA, they were divided into two subgroups according to their therapy into Group AI on biological therapy (15 patients) and Group AII on conventional therapy (15 patients). These in addition to 20 healthy controls (Group B). Results: Basic clinical evaluation and laboratory investigations were done. We found that JIA patients had significantly higher levels of serum neopterin than healthy controls. We also found a highly significant difference between neopterin levels in the activity and remission states among all patients (Group AI and Group AII). Conclusion: We concluded that serum neopterin is a useful marker for cellular immune activation and also indicative of the activity of JIA. Our findings are supported by positive correlations between serum neopterin levels and other markers of activity as TLC, PLT counts, ESR, and CRP. We also concluded that serum neopterin is a sensitive and accurate predictor of disease activity where sensitivity of that test was 93.3% and accuracy was 72.5%. Recommendations: Investigating the serum neopterin measurement in other autoimmune collagen diseases. Assessment the influence of biological therapy on neopterin levels in relation to disease progression.