Ultrasonography was used to examine 42 patients who were complaining of popliteal and suprapatellar pain. When clinically indicated, patients also underwent phlebography, arthrography, and posterior synovectomy. In 17 patients, Baker's cysts were demonstrated by ultrasonogram. Of these 17 patients, four had confirmation of the cysts by arthrography. One patient had simultaneous deep venous thrombosis, proved by phlebography and arthrography. Five patients underwent posterior synovectomy, and the surgical specimens closely correlated with the dimensions predicted by ultrasonography. Internal echoes within the cysts correlated at surgery with either necrotic debris or organized hematoma. To our knowledge, this study is the first series where surgical correlations have been made with the ultrasonographic observations. Ultrasonography of the knee and surrounding area is a useful, painless, and relatively inexpensive method, with several advantages over arthrography.