We aimed to investigate (1) the probable correlation between clinical and ultrasonographic findings in chronic painful primary knee OA patients referred with acute flare-ups and (2) the impact of diagnostic ultrasonography (US) to determine the real source of pain in these patients. We included 100 patients consecutively who were admitted to our outpatient unit with a pain complaint on a single knee with the diagnosis of primary knee OA according to the ACR criteria. The control group consisted of the patients with pain-free knees at least during the last month, who were already included in the study group. The sonographic evaluation of the knee was performed by a physician who was blinded to the clinical evaluation and/or the physical and radiological evaluations. In the present study, sonographic findings were significantly more observed on the painful knees (p < 0.001). The most commonly encountered findings on the symptomatic knees were the suprapatellar effusion (55 %), the baker cyst (25 %), and the pes anserine bursitis. The distribution of the findings on the asymptomatic knees was as follows: 22 %, the suprapatellar effusion and 5 %, the Baker cyst. Effusion was detected in 55 % of the painful knees of our patients with knee OA. This finding was statistically significant compared to the painless knees of the subjects included. The results of our study also showed that there was a significant relation between the Kellgren-Lawrence grading and the frequency of suprapatellar effusion on US examination (p = 0.026). It was concluded that in chronic, primary, painful knee osteoarthritis, US is a valuable diagnostic method in the confirmation of synovitis and/or the inflammatory episode in spite of the absence of obvious clinical parameters. In advanced osteoarthritis, when we consider that the inflammatory episodes are expected findings, the early confirmation of the inflammation on US may be particularly valuable in the clinical setting.