Ultrasound (US), because of its ability to distinguish cystic from solid tissue, is an ideal tool for diagnosing masses in the popliteal fossa. Five cases are presented. Two cases show the typical ultrasound images in the most common popliteal mass, a cyst. A third case significantly points out the necessity of further investigations if the US findings are atypical of a simple cyst and the patient is symptomatic. A fourth case demonstrates that the US image can distinguish an aneurysm of the popliteal artery from a cyst. A fifth case demonstrates that a solid tumor in the popliteal fossa, in contrast to that of a cyst, has a characteristic US image. Ultrasound is a simple, quick, noninvasive outpatient procedure available to almost every physician. In most instances, US appears to be the diagnostic procedure of choice following conventional radiography in the evaluation of swelling in the popliteal fossa.