X-ray measurement of the acromioclavicular (AC) joint may cause difficulties because of different projections and the lack of a reproducible measurement. In order to find the ideal measurement to estimate the state of the AC joint, 28 healthy adult volunteers were X-rayed. The least vulnerable measurement for errors in projection was the distance between the coracoid process and the upper part of the clavicle. The effect of stress was evaluated; the range of normal AC joint laxity was determined as 3 mm.