The aim of this study was to evaluate the effects of different hyaluronic acid (HA) forms on synovial fluid levels of intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) during the treatment of patients with knee osteoarthritis (OA). Forty patients were randomly assigned to 2 groups that were treated with native sodium hyaluronate (group I) or cross-linked hylan G-F 20 (group II). Clinical evaluations and synovial fluid aspirations were performed before the 1st injection (baseline), the 2nd injection (week 1), the 3rd injection (week 2), and at 1 week after the 3rd injection (week 3). Synovial fluid levels of both ICAM-1 and VCAM-1 were significantly reduced at weeks 1 to 3, compared to the baseline values. The Western Ontario and McMaster Universities Osteoarthritis (WOMAC) Index was used for clinical evaluations; the WOMAC pain score and physical function score were progressively improved at weeks 1 to 3 in both groups; the WOMAC stiffness score was significantly improved at week 3 in both groups. No significant differences were noted between the 2 treatment groups in respect to ICAM-1 levels, VCAM-1 levels, WOMAC pain score, stiffness score, or physical function score at any time. The decreased ICAM-1 and VCAM-1 levels after intra-articular HA injection may help to explain the anti-inflammatory effects of HA therapy in knee OA.