Aim of the work: The aim of this study was to evaluate interleukin-23 (IL-23) level in the sera of rheumatoid arthritis (RA) patients and to determine its relation with disease activity and severity. Patients and methods: This study was carried out on 40 RA patients and 40 healthy control subjects. 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 enzymelinked immunosorbent assay. Disease activity score (DAS-28) and rheumatoid arthritis severity scale (RASS) were assessed. Results: Patient’s mean age was 43.3 ± 10.4 years and they were 37 females and 3 males. The mean disease duration of the patients was 4.98 ± 4.1 years (1–15 years) with a mean DAS-28 of 4.8 ± 1.2 (2.4–7.6) and RASS of 41.1 ± 16.9 (16.7–85). The mean IL-23 serum level was significantly higher in RA patients (67.6 ± 39.2 pg/ml) compared to the control (37.7 ± 15.6 pg/ml) (p < 0.001). There were significant correlations between IL-23 levels with the DAS-28 (r = 0.35, p = 0.02), RASS (r = 0.31, p = 0.04), CRP (r = 0.39, p = 0.02), ESR (r = 0.45, p = 0.004), RF (r = 0.48, p = 0.002) and anti-CCP antibodies (r = 0.35, p = 0.04). At a cut-off value of 45 (pg/ml), IL-23 had a sensitivity of 77.8% and a specificity of 75% for detection of active disease and at 43.5 pg/ml the sensitivity was 88.2% and specificity 83.3% for occurrence of physical damage. Conclusion: IL-23 could be a useful marker for disease activity in RA. Its correlation with RASS suggested that IL-23 might be a therapeutic target for prevention of disability.