Objective To investigate the method and the effectiveness of arthroscopy and/or arthrotomy combinedwith postoperative radiotherapy for diffuse pigmented villonodular synovitis (PVNS) of the knee. Methods BetweenSeptember 2000 and August 2010, 97 patients with diffuse PVNS of the knee were treated. There were 38 males and 59 femaleswith a median age of 33 years (range, 8-75 years). The disease duration ranged from 1 week to 30 years, including 52 left kneesand 45 right knees. There were 10 recurrent cases. The extention and flexion of the knee joint were (1.9 ± 2.3)° and (122.9 ± 5.6)°,respectively; the Lysholm score was 43.2 ± 6.7; and the International Knee Documentation Committee (IKDC) score was53.2 ± 5.7, preoperatively. According to the scope and degree of the knee joint lesions, simultaneous anterior and posteriorsynovectomy was performed under arthroscopy in 82 cases, synovectomy under arthroscopy and removal of posterior extraarticularlesion by arthrotomy in 3 cases, synovectomy and the soft tissue lesions resection under arthroscopy in 9 cases, andstaging resection and bone graft in 3 cases. After operation, 76 patients received postoperative radiotherapy. Results Poplitealartery was injuryed in 1 case and the branch of popl iteal veins were injuryed in 3 cases during operation. Intra-articularhemorrhage occurred in 1 case at 3 days after operation. The other patients achieved heal ing of incision by first intentionwithout nerve damage and other complications. All patients were followed up 1 year and 3 months to 11 years and 2 months(median, 61 months) postoperatively. During follow-up, 89 cases had no relapse. At 15 months after operation, the extentionand flexion of the knee joint were (0.2 ± 1.3)° and (135.9 ± 6.6)°, respectively; the Lysholm score was 89.8 ± 5.8; and the IKDCscore was 87.8 ± 5.8. All indexes were significantly improved when compared with the preoperative ones (P lt; 0.05). At 6 monthsto 8 years postoperatively, 8 cases had occurrence, and they had sl ight limitation of the range of motion but had no pain andswelling of the knees after reoperation. Conclusion According to the scope and degree of the knee joint lesions, arthroscopyand/or arthrotomy combined with postoperative radiotherapy should be chosen for diffuse PVNS of the knee so as to obtain good effectiveness. Radiotherapy and enough total radiation dose are important factors to insure no recurrence.