Unilateral knee swelling and pain in young girls may have the following conditions: 1. Chronic injuries, such as meniscal injuries and cruciate ligament injuries, can present with discomfort, pain, and a sense of instability during exercise and swelling after activity. 2, developmental patellar instability, such as patellar subluxation, high patella, etc., during sports, there is pain in the front of the knee, occasionally patellar dislocation. 3, rheumatoid disorders, generally have multi-joint pain, swelling, morning stiffness, abnormalities of ESR, CRP, RF, etc.. Occasionally, there is low fever. 4, metabolic diseases and reactive arthritis are relatively rare. Suggestions: go to a nearby hospital to take frontal and lateral radiographs of both knees, MRI if necessary, and blood tests, most of which can be initially clarified, and then determine further treatment measures.