The aim of this study was to investigate the effects of intra-articular hyaluronic acid (HA) on symptoms, functional outcome, and changes in articular cartilage assessed by magnetic resonance imaging (MRI) in patients with knee osteoarthritis. Thirty patients were randomly assigned to treatment with HA (hylan G-F 20, Synvisc) or saline. The treatment group consisted of 20 patients receiving three weekly injections of HA into one or both knees (30 knees). The control group consisted of ten patients receiving three intra-articular injections of 2 ml saline at the same intervals (ten knees). To determine the effectiveness of the HA therapy, all patients were assessed prior to the injections (baseline) and after the 1st, 2nd, 3rd, and 8th weeks. Assessment comprised the following: pain at rest, at night, and on walking using a visual analogue scale (VAS); Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain, stiffness, and function scores; 15-m walking time; need for analgesics; and evaluation of treatment by the patients. MRI of patellofemoral (PF) articular cartilage was also examined before and after the course of injections at the 8th week. When compared to placebo, a significant statistical difference was found in all clinical parameters. On MRI, although the difference in the PF joint cartilage quality in the HA group before and after the treatment was statistically significant (p < 0.05), this significance was not detected between the groups after the treatment (p > 0.05). After the HA injections, a significant analgesic effect was seen as early as the 3rd week continuing up to the 8th week and functional improvement was seen at the 8th week. In conclusion, intra-articular injections of HA is an effective choice of treatment in patients with knee osteoarthritis.