Purpose: To determine whether it is possible to differentiate between ALPSA(anterior labroligamentous periosteal sleeve avulsion) lesion and Bankart lesion using arthroscopic findings and to investigate the clinical significance of ALPSA lesion. Materials and Methods: This study was performed on 66 cases that underwent arthroscopic Bankart repair for the anterior instability of the shoulder. By the readings of MRI, there were 56 cases(85%) of Bankart lesion and ten cases(15%) of ALPSA lesion. Arthroscopic findings of Each cases were classified and their average follow-up period was 22 months. Results: It was observed that ALPSA lesions developed in younger age groups than Bankart lesions. Under the arthroscope, ALPSA lesions showed various forms, especially there were five cases of severe inferomedial displacements. Within Bankart lesion, 21 cases were observed to be severely displaced and from the arthroscopic findings, it was difficult to distinguish the difference of Bankart lesion cases displaced inferomedially from the neck of scapula without severe lateral displacements and ALPSA lesion. Regarding the recurrence, there were no redislocation but apprehension shows in two cases(20%) with severe inferomedial displacement in ALPSA group. In Bankart lesion, there were four recurrent cases(7.1%); one of redislocation; one case of subluxation; two of apprehension. Conclusion: It was difficult to differentiate Bankart lesion and ALPSA lesion with severe inferomedial displacement and severely displaced ALPSA lesion showed high rate of recurrence. Thus, complete detachment of ALPSA lesion should be performed more carefully to reduce the recurrence rate.