Ultrasound (US) examination of the popliteal space is a noninvasive, reliable technique for the assessment of popliteal cysts. In order to determine the prevalence and significance of popliteal cysts in primary osteoarthritis (OA) of the knee, we conducted a prospective clinical, radiographic and US study on 50 patients and 25 controls. Popliteal cysts were demonstrated by US in 29 of 100 (29%) knees of 21 of 500 (42%) patients, 8 of whom (38%) had bilateral cysts. The majority of cysts were small and symptomless. Cyst rupture and the "pseudothrombophlebitis syndrome" occurred in only 2 of 29 (7%) knees. The occurrence of cysts correlated with the presence of knee effusion and the radiologic grade of OA. The results indicate of high incidence of clinically unsuspected popliteal cysts in patients with symptomatic knee OA. OA may be a more common cause of popliteal cysts than generally recognized.