A pilot study was designed to investigate the efficacy of two different hyaluronic acid preparations combined to physical therapy in patients with knee osteoarthritis in terms of reduction in pain and disability and muscle strengthening. Thirty-seven patients with symptomatic osteoarthritis of the knees were randomly assigned into three groups. First group received a lower molecular weight hyaluronic acid plus physical therapy, second group received a higher molecular weight hyaluronic acid plus physical therapy, and the third group received physical therapy alone. The isokinetic knee muscle strengths and index of severity for osteoarthritis of the knee scores were evaluated at baseline, at the end of treatment (3 weeks) and at 3 months of follow up. At both short-term (3 weeks) and long-term (3 months) evaluations, index of severity for osteoarthritis of the knee scores were reduced in all three groups, while there was no significant muscle strengthening.