We have carried out a scapulometric study, using CT-scan, of 98 shoulders: 36 with recurrent anterior shoulder dislocation (RAD), 37 stable contralateral shoulders (CSS) and 25 normal shoulders (NS). Six parameters were evaluated: Horizontal and Vertical glenohumeral index, glenoid tilt, anteversion angle of the scapula, glenoid angle and humeral retroversion. We found statistically significant differences between the RAD and CSS groups in the horizontal glenohumeral index. Both the RAD and CSS groups showed significant differences in comparison with the NS group in the horizontal glenohumeral index, glenoid tilt and anteversion angle of the scapula. An imbalance of the head-glenoid size and the anterior glenoid tilt are the anatomical factors which favour instability. The determination of these three parameters has great value when assessing patients with anterior shoulder instability. Our results confirm that although the aetiology of anterior glenohumeral instability is multifactorial, there is an anatomical congenital predisposition which favours instability and this predisposition affects (to a lesser extent) the stable contralateral side, confirming the role of subtle congenital dysplasic theory. In addition the following were revealed as relevant parameters in the study of anterior instability: the horizontal glenohumeral index, glenoid tilt and angle of anteversion of the scapula; while the value of the humeral retroversion is under discussion.