Viscosupplementation of the synovial liquid, by intra-articular injection of hyaluronic acid, is a widely used symptomatic treatment in knee osteoarthritis. Besides products designed for multi-injections (typically 3-5 injections at 1-week intervals), special interest is being given to single-injection products that offer specific advantages such as the reduction of the number of visits to the doctor and the number of invasive interventions with their associated risks. However, a question remains about the efficacy of these monoinjections, compared with the multi-injections regimens. A postmarket, prospective, multicenter, open study (ART-ONE 75), was performed with the single-injection product Arthrum 2.5% (3 mL, 75 mg hyaluronic acid) (LCA Pharmaceutical, Chartres, France), on 214 patients with knee osteoarthritis. Patients were followed at 30, 60, 120, and 180 days. The average patient profile at inclusion was age 62.9 years, 56% were women, Kellgren-Lawrence (KL) grade I through III (46% Kellgren-Lawrence status III), body mass index 27.2, and 4 years osteoarthritis anteriority. A post hoc comparison was performed using a single intra-articular injection placebo (326 patients, pooled from 3 randomized controlled trials) providing a similar patient profile. The main criterion was the variation from baseline of the Western Ontario and McMaster Universities Osteoarthritis (WOMAC) Index, pain subscale (A) score (range 0-100), at 60 days, which was reduced by 28.9 (17.4) for the intent-to-treat population (199 patients), 28.0 (17.8) for the per protocol at inclusion population (175 patients), and by 27.7 (16.8) for the per protocol completed population (143 patients). The secondary and accessory criteria included WOMAC A score at the other times, WOMAC B (stiffness) score, WOMAC C (function) score, quality of life, and handicap at each follow-up time. All indexes were significantly improved and continued to improve at the end of the study. The therapeutic assessment at 180 days showed more than 75% of patients were satisfied with pain reduction, mobility improvement, and reduction in taking analgesics and nonsteroidal anti-inflammatory drugs. The percentage of patients defined as Outcome Measures in Rheumatology Clinical Trials-Osteoarthritis Research Society International Standing Committee for Clinical Trials Response Criteria Initiative (OMERACT-OARSI) responders was >86%, from 60 days onward. The overall tolerance was good, without any serious adverse event. The result of the post hoc comparison for the WOMAC A score showed an effect size from 0.33 (95% CI 0.15-0.51) at 60 days to 0.65 (95% CI 0.45-0.85) at 180 days (P < 0.001), versus injected placebo (saline solution), which is clinically relevant in favor of Arthrum 2.5%. The present study suggests the clinical efficacy of a single intra-articular injection of 3 mL intra-articular hyaluronic acid solution containing 75 mg high molecular weight (>2 MDa) native hyaluronic acid.