Background: This study aimed to measure the serum and synovial interleukin (IL) -37 levels in 5 rheumatoid arthritis (RA) patients compared to patients with primary knee osteoarthritis (PKOA) and 6 healthy controls and to detect its relation to RA disease activity. Results: This cross-sectional study 7 included 50 RA patients with a mean age of 40.24±8.62 years, 50 patients with PKOA with a mean age 8 of 56.69±4.21, and 40 healthy controls with a mean age of 41.75 ±7.38 years. The mean serum IL-37 9 level in the RA patients (382.6±73.97 pg/ml) was statistically significantly (p 0.94). Both serum and 12 synovial IL-37 levels were significantly positively correlated with disease activity scores (r= 0.92, P< 13 0.001 and r= 0.85, P< 0.001), tender joint counts (r= 0.83, P< 0.001 and r= 0.82, P< 0.001 ), swollen 14 joint counts (r= 0.72, P< 0.001 and r= 0.60, P< 0.001), visual analogue scores (r=0.82, P< 0.001 and r= 15 0.82, P< 0.001), erythrocyte sedimentation rate (r= 0.75, P