The authors present six cases of cysts of the semilunar cartilages of the knee detected by ultrasonography, all of them confirmed at the time of surgery. Five of these lesions were cysts of the lateral meniscus; one lesion was a cyst of the medial meniscus. Because of ultrasonography, an accurate diagnosis of meniscal cysts is within the reach of the radiologist; it is also possible to visualize the extension of these lesions in a correct way, thereby being of great help to orthopedic surgeons. The main advantages of this technique compared with magnetic resonance imaging (MRI), which is also able to give the same information, are the short time required to perform the examination and the low costs. After outlining their study, the authors give a review of the literature concerning the incidence, clinical features, pathogenesis, and management of the disease.