Introduction:Obesity is a risk factor for knee osteoarthritis (OA). We hypothesised that, because of mechanical overload and metabolic impairment, obese patients with knee OA present more frequent knee extensor system abnormalities compared to lean osteoarthritis patients at high resolution ultrasonography. Methods: Twelve obese patients (Obese group) and a control group of ten lean patients (Lean group) with knee OA were included in this study. Short anteroposterior weight-bearing knee radiographs were taken. Femorotibial alignment and Kellgren-Lawrence score were assessed. Ultrasonographic assessment was performed on the quadriceps and patellar tendons through three scans in both transverse and longitudinal planes, and power-Doppler images were taken. Results: The Obese group presented a trend toward a higher percentage of low degree radiographic knee OA compared to the Lean group. Ultrasonography evidenced the involvement of the peripatellar entheses, synovitis and effusion at the supra-patellar recess and muscular tissue fibroadipous degeneration, and hypotrophy of rectus femoris, particularly evident in the obese group. Conclusions: There are marked abnormalities of the quadriceps and patellar tendons both in obese and lean patients with knee OA. Although obese patients were significantly younger than lean patients, the presence of quadriceps and patellar tendons abnormalities in the knee OA Obese group supports the hypothesis of an early onset of abnormal tendon structure in the younger obese population with knee osteoarthritis. Level of evidence: III.