Interleukin-33 (IL-33) is a member of the IL-1 cytokine family and is associated with the development of different autoimmune diseases as systemic lupus erythematosus (SLE). So, the purpose of this cross-sectional study was to measure the serum IL-33 in children with SLE (c-SLE) in relation to their SLE disease activity index. This study was conducted upon 50 c-SLE patients in comparison to 50 normal matched children as a control group. Disease activity was assessed according to SLE Disease Activity Index (SLEDAI-2K). Serum IL-33 was measured by an Enzyme-linked immunosorbent assay (ELISA). Serum IL-33 was significantly higher in c-SLE patients (median: 157.47, IQR:64.49-237.57ng/l) than controls (median: 10.9, IQR: 10.04-12.51ng/L) (P= 0.001) and negatively correlated with serum C3 and C4 levels. Serum IL-33 levels were significantly higher in high disease activity status (HDAS) patients (SLEDAI-2K ≥ 10) (298.47 ± 78.84ng/l) than lupus low disease activity status (LLDAS) patients (SLEDAI-2K < 10) (112.18 ± 16.23ng/l) (P= 0.001). The receiver operating characteristic (ROC) curve analysis revealed that the best cutoff of serum IL-33 level to predict the disease activity was ≥141.3 ng/l with a sensitivity of 93%, a specificity of 90% and accuracy 97%. We concluded that serum IL-33 was higher in c-SLE patients and positively related to the disease activity index so could be used as a disease activity marker.