The objectives of this study were to assess the ultrasonographic (US) findings in patients with knee osteoarthritis (OA) with pes anserinus tendinitis or bursitis (PATB) syndrome and to determine the correlation between the US findings and the response to local corticosteroid injection. We prospectively studied 26 patients with knee OA with clinically diagnosed PATB syndrome. A linear array 7 MHz transducer was used for US examination of the knee. Seventeen patients were injected locally with tramcinolone acetonide in the anserine bursa area. Response to local corticosteroid injection was evaluated by pain visual analog scale (VAS), Western Ontario and MacMaster (WOMAC) osteoarthritis index and Global patient/physician assessment using Likert scale. On US examination, only 2 patients (8.7%) showed evidence of PATB. Pain VAS, WOMAC pain index and WOMAC physical function index improved significantly after corticosteroid injection. Global patient assessment revealed that 2 patients showed best response, 6 good, 1 fair, 8 the same, and none worse. It is of note that the 2 patients who showed the best response were those who showed US evidence of PATB. This finding shows that US can serve as a useful diagnostic tool for guiding treatment in PATB syndrome of OA patients.