Diffuse pigmented villonodular synovitis (PVNS) is an uncommon aggressive synovial proliferative disorder of unknown etiology affecting the joint linings. Though a histologically benign inflammatory process, because of its aggressive growth with bone destruction or recurrence, it is frequently suggested to occur as a low malignant neoplasm. Optimal treatment is surgery, but the local recurrence rate after radical synovectomy for diffuse PVNS is relatively high due to the infiltrative growth pattern. External beam radiotherapy with moderate doses or intra-articular instillation of radioactive isotopes may improve the likelihood of local control and long-term function in patients with incompletely resected or recurrent diffuse PVNS. I report one case of diffuse PVNS of the right knee joint treated with arthroscopic synovectomy and external beam radiotherapy is presented.