We examined the association between clinical parameters of disease activity and serum hyaluronic acid (HA) level in patients with knee osteoarthritis (OA). Fifty-eight patients with knee OA and 40 healthy individuals were included in this study. Demographic data were noted. The radiological evaluation was done according to Kellgren/Lawrence Grading System. Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) was used in evaluating the clinical disability caused by OA. Erythrocyte sedimentation rate (ESR) and serum C-reactive protein (CRP) level were determined in laboratory evaluations. Serum HA levels (in nanogram per milliliter) of both groups were measured by enzyme-linked immunosorbent assay. The mean ages of patients in OA group and control group were 57.58 (9.55) and 43.3(10.2) years, respectively. There was a significant difference between HA levels of both groups (p = 0.02). Also, there was a significant correlation between HA level and disease duration (p = 0.04), ESR (p = 0.001), and CRP level (p < 0.001). However, there was no significant correlation between HA levels with WOMAC and radiological grade (p > 0.05). There is no satisfactory amount of data regarding HA level in OA. In our study, we demonstrated that HA was in high levels in the serum of patients with OA. We suggest that serum HA levels may be used as a marker to determine the disease activity.