Ultrasonography was performed in 55 patients who had total Charnley hip arthroplasties. Effusions were identified in 19 patients and confirmed in all but 3 by arthrocentesis or at surgery. Aspirations were performed in 5 and demonstrated infection in 2. It is concluded that ultrasound is a valuable noninvasive method for assessing painful hip arthroplasty. It can demonstrate the presence of effusion, which should be aspirated to exclude infection.