This study aimed to determine the ultrasonographic prevalence of Baker's cysts in knees with chronic osteoarthritic pain and investigate for cysts correlates and relationships with scintigraphically established synovitis. Consecutive patients with chronic osteoarthritic knee pain underwent clinical examination, X-rays, ultrasonography and early-phase bone scintigraphy. Eighty-nine Baker's cysts were detected in 328 knees with chronic osteoarthritic pain (27%), whereas one cyst was identified among the 54 non-osteoarthritic knees (2%, P<0.001). Baker's cysts were detected in 72/195 (37%) patients with knee osteoarthritis. Abnormal and intense tracer accumulation in early-phase bone scintigraphy were significantly more frequent in osteoarthritic knees with Baker's cysts (97 and 56%, respectively), than in those without (89 and 40%, respectively, P<0.05 for both). Clinical and radiographic variables could not predict the presence of those cysts. Baker's cysts are a common ultrasonographic finding in knees with chronic osteoarthritic pain and are associated with synovial inflammation and its grade.