Classification of glenohumeral instability is confusing. We think that the existence of trauma, directions of instability, voluntarism, and other factors make classification difficult. The purpose of this article is to create a new classification. One hundred eighty-nine patients with glenohumeral instability involving 207 joints (mean patient age 21.5 years) were subjects of this investigation. Our new classification, which is composed of three main factors (level of trauma, direction of instability, and voluntarism) and some subfactors, simplified it quite well. Abbreviations also make it easier to indicate each joint's condition. About half the subjects had no trauma or mild trauma. Two thirds of the joints with more than one dislocation or subluxation showed instability in other directions in addition to the direction of dislocation or subluxation. This classification is very useful to compare pathogenesis and results of treatment in patients with glenohumeral instability.