Osteoarthrosis of the knee is a degenerative disease with inflammatory episodes objectively shown by arthrosonography. Cartilage Oligomeric Matrix Protein - COMP is a useful biomarker for early cartilage destruction. The aim of this paper is a comparative analysis of the clinical, arthrosonographic findings and the COMP concentration in the sera of patients for the detection of joint inflammation. The analysis included 88 patients with knee OA. Clinical examination determined the outflow, arthrosonography the size of synovitis and effusion, and serum analysis the COMP concentration (ng/ml). Minimum outflow had 34.1% of patients, moderate 22.7%, and significant 4.5%. Sensitivity of the clinical diagnosis of outflow is 73%, and specificity 73% (p=0.000). Seventy five percent of patients had effusion; 28.4% of patients in the suprapatellar recessus (SR), 27.3% in the medial (MR), and 62.5% in the lateral (LR). In SR, effusion was 10.13±4.35 mm, MR 8.53±2.27 mm, LR 11.38±4.44 mm. Synovitis was found in 67% of patients, in SR the size of 4.84±3.57 mm, in MR 3.15±1.86 mm, in LR 6.09±2.80 mm. The average value of the size of effusion in patients with significant outflow in SR was 13.85 (10.36-17.43) mm (p=0.000), MR 4.9 (0-10.22) mm (p=0.008), LR 12.0 (11.34-14.50) mm (p=0.000), in LR with moderate outflow 6.94 (1.16-8.13) mm and minimum outflow 4.9 (0-7.25) mm. There is a significant correlation between the size of synovitis and effusion in the SR, MR and LR (p=0.000). The average value of the concentration of COMP in patients without effusion was 54 (44.5-58) ng/ml, with effusion 57 (48.75-64.25) ng/ml (p=0.030). Arthrosonography and the determination of the COMP concentration are sensitive methods for diagnosing joint effusion.