There is still discussion concerning the methods for treating Tossy type 3 dislocations of the acromioclavicular joint. Since 1995, the authors have treated 10 patients of type 3 dislocations by arthroscope-assisted modified Weaver and Dunn operation with favorable results. The operation consisted of diagnostic shoulder arthroscopy, arthroscopic resection of acromial end of coracoacromial ligament with bone block, excision of distal end of clavicle, bone block transfer of coracoacromial ligament into the medullary canal of clavicle, and augmentation between coracoid process and resected distal end of clavicle with the Mersilene tape. The advantages of this arthroscope-assisted modified Weaver and Dunn operation are as follows: (1) Using the shoulder arthroscope, associated patholgy in the shoulder joint can be found and treated appropriately. (2) Arthroscopic resection of the acromial end of coracoacromial ligament can give the small incision and least damage to the deltoid muscle so that immediate post-operative range of motion exercise can be possible. (3) Bone block transfer of coracoacromial ligament and augmentation between coracoid process and resected clavicular end can prevent displacement of the resected clavicular end.