Shoulder pain due to compression of the axillary nerve by fibrous bands in the quadrilateral space has been termed the quadrilateral space syndrome. Selective atrophy of the teres minor muscle, which is innervated by the axillary nerve, was demonstrated at magnetic resonance imaging in three patients with clinical findings of the syndrome. This finding, in the appropriate clinical setting, is highly suggestive of the quadrilateral space syndrome, which is a potentially reversible cause of shoulder pain.