Many cystic lesions occur around the knee and may produce overlapping clinical features, rendering the clinical diagnosis difficult. A 50-year-old woman presented with a soft tissue swelling on the medial aspect of her right knee. The diagnosis of pes anserine bursitis was made, based on typical MR imaging features. Cystic masses occurring in and around the knee, such as bursae and recesses, meniscal and ganglion cysts, and benign and malignant soft tissue masses that may mimic cysts, are classified and described. The role of MR imaging in making an accurate diagnosis and distinguishing among the various masses is discussed.