Twenty-nine patients (mean age, 61 years) received three intra-articular injections of 10 mg of hyaluronic acid in 1 ml of phosphate buffer; the injections were given at three-day intervals. Six of the patients had osteoarthritis, six had Duplay's disease, and 17 had both osteoarthritis and Duplay's disease. The patients were evaluated after each injection and 11 days after the last injection. On all measures of treatment effectiveness, including joint mobility, pain, and analgesic consumption, the patients showed rapid and significant improvement. No side effects were reported.