Glenoid shape is closely related to shoulder stability and its abnormality is considered to affect the head position in shoulders with atraumatic instability. However, it still is unclear how the glenoid shape in shoulders with atraumatic instability is different from the glenoid shape in normal shoulders. The current authors investigated glenoid shape of 45 healthy individuals (20 males and 25 females ; 22 years on average) and 20 patients with atraumatic posterior instability with multidirectional laxity (six males and 14 females ; 19 years on average) using three-dimensional magnetic resonance imaging. The tilting angles of the glenoid bone were measured in five consecutive planes perpendicular to the long axis of the glenoid and cross sections were divided into three types (concave, flat, convex). In healthy individuals, the average tilting angles from the bottom to the top of the glenoid bone were 3.0±3.6°, 1.0±3.2°, -1.0±2.0°, -2.3±3.9°, -6.9±3.7° anteriorly, and those of patients were -6.1±4.0°, -4.0±3.6°, -4.8±3.2°, -5.5±2.7°, and -7.5±3.1°. The type of cross section also was different on the bottom plane where the concave shape accounted for 78% of healthy cases while it accounted for 0% of patients. The loss of tilting angles and concavity of the inferior glenoid would correlate with the direction of the head translation in posterior instability.