Background: Rheumatoid arthritis (RA) is a chronic autoimmune inflammatory disease. The pathogenesis of RA is mediated by an interdependent network of cytokines which has been extended to include the cytokine, IL-23.         IL-23 production appears to be of great importance in the inflammatory reaction in RA. Objectives: We aimed in this study to estimate interleukin-23 (IL-23) level in the sera of rheumatoid arthritis (RA) patients and to determine its relation with disease activity. Subjects and methods: This study was carried out on 40 patients with RA, and 40 healthy control subjects. RA disease activity was measured by 28-joint disease activity score (DAS-28). All patients were subjected to full history taking, thorough clinical examination, radiological and laboratory investigations including c- reactive protein (CRP), erythrocyte sedimentation rate (ESR), rheumatoid factor (RF), anti cyclic-citrullinated peptide (anti-CCP) antibodies. Serum IL-23 was measured by enzyme-linked immunosorbent assay (ELISA). Results: In RA patients serum IL-23 level was significantly elevated in comparison to the healthy controls (p<0.001). There was a significant positive correlations between IL-23 level and DAS 28 score. The highest level was detected in patients with high disease activity (p=0.03). There was statistically significant correlation between IL-23 levels and ESR, CRP, RF, anti-CCP antibodies. Conclusion: IL-23 could be a useful marker for disease activity in RA.