Aim of the work To compare the expression of serum interleukin 33 (IL33), a new member of the interleukin1 (IL-1) cytokine family, in rheumatoid arthritis (RA) versus osteoarthritis (OA) patients and to correlate it with clinical, laboratory and radiographic parameters. Subjects and methods 20 RA and 20 primary knee OA patients. The levels of serum IL-33 were measured by enzyme-linked immunosorbent assay (ELISA) while anticyclic citrullinated peptide (Anti-CCP), rheumatoid factor (RF), erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) were measured by standard laboratory techniques. Plain x-ray of both hands and wrists were evaluated using the modified Larsen score 1995 (MLS) in RA patients. Knee OA grading was performed according to the KellgrenLawrence classification. The correlation of IL-33 level with clinical, laboratory and radiological data of RA and OA was analyzed. Results Serum IL-33 level was significantly higher in RA than in OA patients (P