The objective of this 12-week, double-masked, randomized, multicenter study was to compare the elastoviscous properties of a high-molecular-weight viscosupplement, hylan G-F 20 (polymer concentration, 0.8%), with those of a lower-molecular-weight hyaluronan (LMW HA) product (polymer concentration, 1%) and to determine the relationship of elastoviscosity to efficacy in the treatment of patients with osteoarthritis (OA) of the knee. Patients had radiographically confirmed primary idiopathic OA of the knee (Larsen grades I to V) with pain despite other treatments. After a 2-week washout period, 70 patients (73 knees) received three 2-mL intra-articular injections of test solution at 1-week intervals. Thirty-eight patients (38 knees) received hylan G-F 20, and 32 patients (35 knees) received LMW HA. During the 12-week follow-up period, the primary outcome measures assessed by patients (using a visual analogue scale) were weight-bearing pain, most painful knee movement, and overall treatment response; the primary outcome measures assessed by study evaluators were weight-bearing pain and overall assessment of treatment. The dynamic elastoviscous properties of the test solutions were measured on an oscillating Couette-type rheometer. Hylan G-F 20 was more elastoviscous than the LMW HA at all frequencies measured (0.001 to 10 Hz). At the final evaluation, patients who received hylan G-F 20 had significantly better results on all primary outcome measures compared with those who received LMW HA. No systemic adverse events were reported. Local adverse events consisted of pain or swelling, noted in 2 of 38 knees injected with hylan G-F 20, and pain, noted in 1 of 35 knees injected with LMW HA (adverse event rates per injection, 1.8% and 0.9%, respectively). The difference in the incidence of adverse events between groups was not statistically significant. The higher-molecular-weight, more elastoviscous hylan G-F 20 had significantly greater pain-relieving effects than did the lower-molecular-weight, less elastoviscous hyaluronan.