A 79-year-old woman presented to the emergency department with right posterior knee pain. One day before her visit, she experienced abrupt, intense pain in the right popliteal fossa when getting up from the sofa. The pain was worsened by flexing the knee. She denied cough, dyspnea, and chest pain. Physical examination revealed swelling in the popliteal region with ecchymosis in the popliteal fossa accompanied by a palpable mass (Figure 1A). Ultrasonography visualized a tender cyst below the ecchymosis (Figure 1B), but there were no findings of deep vein thrombosis.