A prospective, multicenter, randomized, double-blind, controlled trial was conducted in 226 patients with knee osteoarthritis to evaluate the safety and efficacy of intraarticular injections of sodium hyaluronate. Patients were randomized to three weekly injections of 30 mg sodium hyaluronate or physiologic saline (control) and were observed for an additional 25 weeks. In comparison with the control group, among patients who completed at least 15 weeks of the study and whose Western Ontario and McMaster Universities Osteoarthritis Index pain score for the contralateral knee was less than 12 at baseline, sodium hyaluronate injection resulted in improvement in Western Ontario and McMaster Universities Osteoarthritis Index pain score, patient and investigator global assessments, and pain on standing from Weeks 7 to 27. Fifty-eight percent of patients treated with sodium hyaluronate achieved a 5-unit or greater improvement in mean pain score from Weeks 7 through 27, compared with 40% of control patients. In addition, nearly twice as many patients treated with sodium hyaluronate as with saline (30% versus 17%, respectively) achieved a net improvement of at least 7 units. In contrast to treatment with saline, Western Ontario and McMaster Universities Osteoarthritis Index pain score for the contralateral knee was inversely related to the magnitude of improvement after treatment with sodium hyaluronate. Few side effects were attributed to treatment, and no differences between treatment groups were seen in this respect (sodium hyaluronate, nine [8%]; saline, 11 [10%]). The incidence of injection site reactions was low (sodium hyaluronate, 1.2 %; saline, 1.5%). The results indicate that sodium hyaluronate treatment is well tolerated and produces statistically and clinically significant improvement of symptoms in patients with mild to moderate knee osteoarthritis in whom pain in the contralateral knee is relatively modest.