OBJECTIVE: To investigate the ultrasonographic findings in knee OA patients and to examine the possible causes of pain in osteoarthritis by ultrasonography. METHODS: Ultrasonography was performed with 7.5 MHz linear probe in 64 knee OA patients who fulfilled the ACR criteria. All patients were graded according to the Kellgren-Lawrence grades and then classified into group 1 (K/L I and II) and Group 2 (K/L III and IV). Also WOMAC score, BMI, laboratory finding (ESR, CRP) were checked. Ultrasonographic findings was examined; effusion, thickening of synovium, vertical length of medial and lateral osteophyte (longitudinal view), length of capsular distension (medial longitudinal view), evidence of bursitis and articular cartilage. RESULTS: 50.0% of patients had effusion, among whom 68.7% patients also had synovial thickening. In all patients, the severity of pain was correlated with 4 variables; the presence of effusion, disease duration, the length of medial osteophyte, the length of capsular distension (r=0.279, r=0.415, r=0.537, r=0.608, respectively, p<0.05). The length of medial osteophyte, the degree of capsular distension and disease duration were significantly correlated with WOMAC pain score in Group 1 (p<0.05). After multiple regression analysis, the length of medial osteophyte alone had correlation with the pain severity in Group 1 (r2= 0.396 p<0.05) and the only length of capsular distension was significantly correlated with WOMAC pain score in Group 2 (r=0.609, p<0.05). CONCLUSION: The length of osteophyte may be more related with pain severity in mild cases (K/L score I and II) while capsular distension could be an important factor causing knee pain in more advanced knee OA (K/L score III and IV).