45 patients with knee osteoarthritis were randomized following arthroscopic debridement to receive either weekly intra-articular injections of high molecular weight hyaluronic acid (Hylan G-F 20, Synvisc), low molecular weight hyaluronic acid (Hyalgan) or to a control group that did not receive intra-articular injection. The purpose of the study was to compare the effects of hyaluronic acid following arthroscopic debridement on clinical outcome, assessed using the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). Secondarily, the relative efficacy of low- and high-molecular weight HA was evaluated. As preoperative scores were imbalanced across treatment groups, change in WOMAC score from baseline was assessed. There was no significant difference between groups at 6 months postoperatively, though 12 month results indicated significantly greater reduction in WOMAC scores in the hyaluronic acid groups compared to the control group. No significant difference was noted between the high- and low-molecular weight HA groups.