Ultrasound scanning techniques detected popliteal cysts in 14 of 24 knees affected with rheumatoid arthritis and associated with an anterior effusion. Serial scans demonstrated the persistence of cysts when effusion was not controlled by intraarticular steroid and lidocaine, and regression of cyst following control of effusion with anterior synovectomy. As the technique is noninvasive, painless, and reproducible, ultrasound scanning should be the technique of choice for the detection and assessment of popliteal cysts.