Purpose: The purpose of this study was to know the cause of the tenderness at pes anserinus. Materials and Methods: Out of 24 patients with tenderness at pes anserinus, 23 patients were female and 1 patient was male, and their average age was 65.9 years old. We checked the tenderness at pes anserinus by history taking & physical examniation and then, checked the pes anserinus for the presence of bursitis by US in outpatient clinic. With no evidence of bursitis by US, we injected steroid-lidocaine mixture intraarticularly and checked whether the tenderness disappeared after 2~3 minutes. Results: There was no case with bursitis at pes anserinus by US. The tenderness at pes anserinus diminished in 2-3 minutes after the intraarticular injection of the steroid-lidocaine mixture. After 6 weeks follow up, 16 patients(66.7%) had little or no tenderness at pes anserinus. 8 cases had the recurrence of tenderness recurred, 4 cases underwent arthroscopic operation on the meniscal tear, and 1 case underwent total knee arthroplasty. All cases underwent any operations had the tenderness at pes anserinus disappeared. The others 3 cases did not undergo total knee arthroplasty despite of radiologic obliteration of knee joint adequate for Kellgren-Lawrence grade IV. Conclusion: Without the bursitis at Pes Anserinus, patients the osteoarthritis may have the tenderness due to the referred pain.