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The aim of the study was to investigate the effect of hyaluronic acid (HA) intra articular injections (IA) on osteoarthritis (OA) biomarkers in patients with knee OA. Prospective open label study. Fifty-one patients with unilateral symptomatic K-OA received IA injections of 2mL of HA on days (D) 1, 7, 14 and were followed 3 months. At D-15 patients were examined and X-rays performed, to exclude patients with bilateral K-OA, or those with more than three symptomatic OA joints. From 15 days (D-15) before the first injection to D90 concomitant therapies were unchanged. Walking pain (WP) on VAS was obtained at each visit. Urine (U) and serum (S) samples were obtained at D-15, D1, D30, and D90. S-C2C, S-Cartilage oligomeric matrix protein, S-HA, S-CS 846 epitope, S-type II collagen propeptide, and U-type II collagen C telopeptide (U-CTX II/creatinin) were assayed. Predictive factors of response were analyzed using logistic regression. Correlations between variables were obtained using Spearman test. Forty-five patients were analyzed. Between D-15 and D1 there was no difference for any biomarkers At D1, WP (SD) was correlated with U-CTX II/creat (p = 0.006). Between D1 and D90: U-CTX II/creat decreased significantly. After adjustment for confounding variables there was a significant correlation between clinical response and U-CTX II/creat variation. U-CTX II and S-HA at baseline were independently predictive of clinical response. This study showed that 90 days after HA IA injections, U-CTX II levels significantly decrease compared to baseline, suggesting a slowdown of type II collagen degradation. © 2011 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 30:679–685, 2012