The objectives of the present work were (1) to establish the prevalence of the abnormalities detected by magnetic resonance imaging (MRI) and ultrasonography (US); and (2) to compare these imaging techniques in detail. The study group consisted of 58 patients with symptomatic knee OA and 16 volunteer control subjects. Knee joint was evaluated for femoral condylar cartilage changes, effusion, synovial thickening and popliteal cysts using MRI and US. All knees with OA had cartilage abnormalities on US examinations and normal cartilage was detected in less than 3% of these knees by MRI. Majority of the knees with OA had effusion using US (70%) or MRI (85%). Synovial thickening observed on US (34%) and MRI (50%) were common in the knees with OA. Popliteal cysts were detected in 40% of the knees with OA using US and 35% using MRI. This study confirmed that there was a significant correlation between the MRI and US techniques for evaluating the cartilage and soft tissue changes in the patients with knee OA. There were more significant differences between the controls and the symptomatic knees which had Kellgren-Lawrence (K-L) grade 2 or more OA for the cartilage and soft tissue abnormalities on MRI and US. The prevalence of cartilage changes, effusion, synovial thickening and popliteal cyst using MRI and US were increased as the radiographic grade of OA increased. US examinations could be an alternative to initial evaluation tool to MRI in patients with knee OA.