Acromioclavicular (AC) sprains can be graded in 3 to 6 different types according to Tossy or Rockwell, respectively. In mild sprains (Tossy and Rockwell I), movements in the AC joint are minimal, because the coraco-clavicular ligaments are intact. In these patients, stress radiography is usually normal, and sonographic examination at rest can be normal as well, showing minimal or no displacement between the 2 extremities of the bones. We present a simple dynamic maneuver to enhance the diagnosis of these mild sprains known as the cross-arm maneuver, in which the hand is placed on the opposite shoulder. The dynamic sonographic examination during this maneuver clearly shows abnormal movements in the clavicle's extremity, which "falls down" to the acromion in the cross-arm position and is raised and pulled from the acromion at rest. The maneuver is very easy to perform and may be useful when a mild AC joint sprain is suspected.