The purpose of this study was to evaluate prospectively the findings during shoulder arthroscopy in patients with recurrent anterior instability of the glenohumeral joint. One hundred and seventy-eight patients who fulfilled the criteria of having had at least one documented shoulder dislocation were included in the study. The pathological findings most frequently noted at arthroscopy were: anterior glenoid labral tears (85%), ventral capsule insufficiency (80%), Hill-Sachs compression fractures (67%), glenohumeral ligament insufficiency (55%), rotator cuff tears (20%), posterior glenoid labral tears (8%), and SLAP lesions (5%). Abnormalities were noted more frequently than expected, and there were significant differences between preoperative and postoperative diagnoses. Our study has taught us that a multiplicity of morphological changes are associated with instability of the glenohumeral joint, and that there is no single cause. The labrum and rim of the anterior inferior glenoid, for instance, showed typical abnormalities corresponding to different entities of anterior instability. In practice, this is very important, as the abnormalities visualized by imaging methods determine the surgical treatment.