OBJECTIVES: We report methodology and results of a post-launch study which was commissioned by French Health Authorities in order to measure the effectiveness and cost of Suplasyn(r), a hyaluronic acid indicated in the treatment of knee osteoarthritis, in real life conditions. METHODS: An observational, multicentric, prospective, longitudinal study was designed in order to compare pain status, functional capacity, quality of life and cost in patients with knee osteoarthritis, before and after treatment with Suplasyn(r). Patients should meet the American College of Rheumatology criteria for osteoarthritis and be currently treated by analgesics or nonsteroidal anti-inflammatory drugs (NSAIDs). The decision to treat with Suplasyn(r) was left to the investigators’ discretion. Treated patients were followed retrospectively during three months and prospectively during six months. Main evaluations were performed at initiation (V0) and at three (V4) and six months (V5). Cost were compared over three 3-month periods, P1 (before Suplasyn(r) injection), P2 (V0-V4) and P3 (V4-V5). They were computed in the perspective of public health insurance. RESULTS: In total, 296 patients were included. Mean age was 68.67 ± 10.26 years. The main clinical criterion, the WOMAC score, showed a significant improvement for both functional and pain subscales over the time: scores at V4 and V5 were significantly reduced compared to V0 (p<0.0001). The SF-12 Quality-of-Life scale was significantly improved for “physical” (p<0.0001) and “mental” (p<0.0001) dimensions. Between P1 and P3, the proportions of patients receiving NSAIDs and Proton Pump Inhibitors fell respectively from 68.6% to 29.1%, and from 19.3% to 8.7% (p<0.0001). Finally, the average medical cost per patient decreased from 233.59 EUR (IC95%: 202.14-265.04) in P1 to 205.64 (IC95%: 191.70-219.57) in P2 and to 145.20 (IC95%: 77.36-213.05) in P3. CONCLUSIONS: In real life conditions, Suplasyn(r) significantly improves function, pain and quality-of-life of patients with knee osteoarthritis and reduces medical treatment cost.