Objective To evaluate the feasibil ity and effectiveness of allograft osteochondral transplantation with arthroscopic assistance for osteochondritis dissecans (OCD) associated with large osteochondral defects. Methods From January 2004 to May 2007, 13 patients with OCD with large osteochondral defects were treated. There were 7 males and 6 females, aged 18-59 years with an average of 35.8 years. The locations were left side in 8 cases and right side in 5 cases. The disease course was 7 days to 20 years with the median duration of 42 months. Four cases had obvious sprained history. The involved locations were lateral portion of the medial femur condyle (MFC) in 5 cases, thochlea area of MFC in 2 cases and lateral femur condyle in 6 cases. The range of motion was (95.0 ± 13.5)° and the Lysholm score was 62.23 ± 7.79. According to International Cartilage Repair Society classification system and the Guhl classification of OCD under arthroscopy, all the patients were type IV. Defect areas were 3-7 cm2 with an average of 4.32 cm2. The depths of defects were 0.8-2.0 cm with an average of 1.55 cm. Allograft osteochondral transplantation combining with adsorbable screw was appl ied for the lesions. The patients were closely instructed to do exercise through following up. Results The wounds all healed by first intention Thirteen cases were followed up for 2.2 years (14 months to 4 years) after operation. Three months after operation, 2 cases had pain after continuous exercise for 1 hour, they were directed in the methods of functional exercise and to strengthen their quadriceps femoris. Medicine was given in 2 patients who had sign of friction feel ing of joint 6 months after operation. The range of motion at the last follow-up was (137.0 ± 9.8)°, showing statistically significant difference when compared with that before operation (P lt; 0.05). The X-rays and EMRI examinations at following-up indicated that bone healed well and articular facet was integrity. The Lysholm score was 92.08 ± 7.64 one year after operation, showing statistically significant difference when compared with that before operation (P lt; 0.05). Conclusion Allograft osteochondral transplantation with arthroscopic assistance is a useful method in treatment of OCD with large osteochondral defects of the knees.